TRANSCRIBER'S NOTE:
There are a large number of compound words in this book including bird names which occur joined, spaced and hyphenated. No attempt has been made to correct these discrepancies as these are mostly alternative spellingd of thw same word. In the case of bird names it is difficult to decide as ornithologists are still debating on this subject.

TERRITORY IN BIRD LIFE

TERRITORY IN
BIRD LIFE

BY H. ELIOT HOWARD

WITH ILLUSTRATIONS BY
G. E. LODGE AND H. GRÖNVOLD

NEW YORK
E. P. DUTTON AND COMPANY
1920

PREFACE

When studying the Warblers some twenty years ago, I became aware of the fact that each male isolates itself at the commencement of the breeding season and exercises dominion over a restricted area of ground. Further investigation, pursued with a view to ascertaining the relation of this particular mode of behaviour to the system of reproduction, led to my studying various species, not only those of close affinity, but those widely remote in the tree of avian life. The present work is the outcome of those investigations. In it I have endeavoured to interpret the prospective value of the behaviour, and to trace out the relationships in the organic and inorganic world which have determined its survival. Much is mere speculation; much with fuller knowledge may be found to be wrong. But I venture to hope that a nucleus will remain upon which a more complete territorial system may one day be established.

I have to thank Mr. G. E. Lodge and Mr. H. Grönvold for the trouble they have taken in executing my wishes; I also want to record my indebtedness to the late E. W. Hopewell; and to Professor Lloyd Morgan, F.R.S., I am beholden more than I can tell.

CONTENTS

LIST OF ILLUSTRATIONS

SCIENTIFIC NAMES OF BIRDS
MENTIONED IN THE TEXT

TERRITORY IN BIRD LIFE

CHAPTER I

INTRODUCTION

In his Manual of Psychology Dr Stout reminds us that "Human language is especially constructed to describe the mental states of human beings, and this means that it is especially constructed so as to mislead us when we attempt to describe the working of minds that differ in a great degree from the human."

The use of the word "territory" in connection with the sexual life of birds is open to the danger which we are here asked to guard against, and I propose, therefore, before attempting to establish the theory on general grounds, to give some explanation of what the word is intended to represent and some account of the exact position that representation is supposed to occupy in the drama of bird life.

The word is capable of much expansion. There cannot be territories without boundaries of some description; there cannot well be boundaries without disputes arising as to those boundaries; nor, one would imagine, can there be disputes without consciousness as a factor entering into the situation; and so on, until by a simple mental process we conceive of a state in bird life analogous to that which we know to be customary amongst ourselves. Now, although the term "breeding territory," when applied to the sexual life of birds, is not altogether a happy one, it is difficult to know how otherwise to give expression to the facts observed. Let it then be clearly understood that the expression "securing a territory" is used to denote a process, or rather part of a process, which, in order to insure success to the individual in the attainment of reproduction, has been gradually evolved to meet the exigencies of diverse circumstances. Regarded thus, we avoid the risk of conceiving of the act of securing a territory as a detached event in the life of a bird, and avoid, I hope, the risk of a conception based upon the meaning of the word when used to describe human as opposed to animal procedure.

Success in the attainment of reproduction is rightly considered to be the goal towards which many processes in nature are tending. But what is meant by success? Is it determined by the actual discharge of the sexual function? So many and so wonderful are the contrivances which have slowly been evolved to insure this discharge, that it is scarcely surprising to find attention focused upon this one aspect of the problem. Yet a moment's reflection will show that so limited a definition of the term "success" can only be held to apply to certain forms of life; for where the young have to be cared for, fostered, and protected from molestation for periods of varying lengths, the actual discharge of the sexual function marks but one stage in a process which can only succeed if all the contributory factors adequately meet the essential conditions of the continuance of the species.

Securing a territory is then part of a process which has for its goal the successful rearing of offspring. In this process the functioning of the primary impulse, the acquirement of a place suitable for breeding purposes, the advent of a female, the discharge of the sexual function, the construction of the nest, and the rearing of offspring follow one another in orderly sequence. But since we know so little of the organic changes which determine sexual behaviour, and have no means of ascertaining the nature of the impulse which is first aroused, we can only deal with the situation from the point at which the internal organic changes reflect themselves in the behaviour to a degree which is visible to an external observer. That point is reached when large numbers of species, forsaking the normal routine of existence to which they have been accustomed for some months, suddenly adopt a radical change in their mode of behaviour. How is this change made known to us? By vast numbers of individuals hurrying from one part of the globe to another, from one country to another, and even from mid-ocean to the coasts; by detachments travelling from one district to another; by isolated individuals deserting this place for that; by all those movements, in fact, which the term migration, widely applied, is held to denote. Now the impulse which prompts these travelling hosts must be similar in kind whether the journey be long or short; and it were better, one would think, to regard such movements as a whole than to fix the attention on some one particular journey which fills us with amazement on account of the magnitude of the distance traversed or the nature of the difficulties overcome. For, after all, what does each individual seek? There may be some immature birds which, though they have not reached the necessary stage of development, happen to fall in with others in whom the impulse is strong and are led by them—they know not where. But the majority seek neither continent nor country, neither district nor locality is their aim, but a place wherein the rearing of offspring can be safely accomplished; and the search for this place is the earliest visible manifestation in many species of the reawakening of the sexual instinct.

The movements of each individual are then directed towards a similar goal, namely, the occupation of a definite station; and this involves for many species a distinct change in the routine of behaviour to which previously they had been accustomed. Observe, for example, one of the numerous flocks of Finches that roam about the fields throughout the winter. Though it may be composed of large numbers of individuals of different kinds, yet the various units form an amicable society actuated by one motive—the procuring of food. And since it is to the advantage of all that the individual should be subordinated to the welfare of the community as a whole there is no dissension, apart from an occasional quarrel here and there.

In response, however, to some internal organic change, which occurs early in the season, individuality emerges as a factor in the developing situation, and one by one the males betake themselves to secluded positions, where each one, occupying a limited area, isolates itself from companions. Thereafter we no longer find that certain fields are tenanted by flocks of greater or less dimensions, while acres of land are uninhabited, but we observe that the hedgerows and thickets are divided up into so many territories, each one of which contains its owner. This procedure, with of course varying detail, is typical of that of many species that breed in Western Europe. And since such a radical departure from the normal routine of behaviour could scarcely appear generation after generation in so many widely divergent forms, and still be so uniform in occurrence each returning season, if it were not founded upon some congenital basis, it is probable that the journey, whether it be the extensive one of the Warbler or the short one of the Reed-Bunting, is undertaken in response to some inherited disposition, and probable also that the disposition bears some relation to the few acres in which the bird ultimately finds a resting place. Whilst for the purpose of the theory I shall give expression to this behaviour in terms of that theory, and speak of it as a disposition to secure a territory, using the word disposition, which has been rendered current in recent discussion, for that part of the inherited nature which has been organised to subserve a specific biological purpose—strict compliance with the rules of psychological analysis requires a simpler definition; let us therefore say "disposition to remain in a particular place in a particular environment."

But even granting that this disposition forms part of the hereditary equipment of the bird, how is the process of reproduction furthered? The mere fact of remaining in or about a particular spot cannot render the attainment of reproduction any less arduous, and may indeed add to the difficulties, for any number of individuals might congregate together and mutually affect one another's interests. A second disposition comes, however, into functional activity at much the same stage of sexual development, and manifests itself in the male's intolerance of other individuals. And the two combined open up an avenue through which the individual can approach the goal of reproduction. In terms of the theory I shall refer to this second disposition as the one which is concerned with the defence of the territory.

Broadly speaking, these two dispositions may be regarded as the basis upon which the breeding territory is founded. Yet inasmuch as the survival value of the dispositions themselves must have depended upon the success of the process as a whole, it is manifest that peculiar significance must not be attached to just the area occupied, which happens to be so susceptible of observation; other contributory factors must also receive attention, for the process is but an order of relationships in which the various units have each had their share in determining the nature and course of subsequent process, so that, as Dr Stout says, when they were modified, it was modified.

Now the male inherits a disposition which leads it to remain in a restricted area, but the disposition cannot determine the extent of that area. How then are the boundaries fixed? That they are sometimes adhered to with remarkable precision, that they can only be encroached upon at the risk of a conflict—all of this can be observed with little difficulty. But if we regard them as so many lines definitely delimiting an area of which the bird is cognisant, we place the whole behaviour on a different level of mental development, and incidentally alter the complexion of the whole process. It would be a mistake, I think, to do this. Though conscious intention as a factor may enter the situation, there is no necessity for it to do so; there is no necessity, that is to say, for the bird to form a mental image of the area to be occupied and shape its course accordingly. The same result can be obtained without our having recourse to so complex a principle of explanation, and that by the law of habit formation. In common with other animals, birds are subject to this law in a marked degree. An acquired mode of activity becomes by repetition ingrained in the life of the individual, so that an action performed to-day is liable to be repeated to-morrow so long as it does not prejudice the existence or annul the fertility of the individual.

Let us see how this may have operated in determining the limits of the area acquired, and for this purpose let us suppose that we are observing a male Reed-Bunting recently established in some secluded piece of marsh land. Scattered about this particular marsh are a number of small willows and young alder trees, each one of which is capable of providing plenty of branches suitable for the bird to perch upon, and all are in a like favourable position so far as the outlook therefrom is concerned. Well, we should expect to find that each respective tree would be made use of according to the position in which the bird happened to find itself. But what actually do we find—one tree singled out and resorted to with ever-increasing certainty until it becomes an important point in relation to the occupied area, a headquarters from which the bird advertises its presence by song, keeps watch upon the movements of its neighbours, and sets out for the purpose of securing food. We then take note of its wanderings in the immediate vicinity of the headquarters, especially as regards the direction, frequency, and extent of the journeys; and we discover not only that these journeys proceed from and terminate in the special tree, but that there is a sameness about the actual path that is followed. The bird takes a short flight, searches a bush here and some rushes there, returns, and after a while repeats the performance; we on our part mark the extreme limits reached in each direction, and by continued observation discover that these limits are seldom exceeded, that definition grows more and more pronounced, and that by degrees the movements of the bird are confined within a restricted area. In outline, this is what happens in a host of cases. By repetition certain performances become stereotyped, certain paths fixed, and a routine is thus established which becomes increasingly definite as the season advances.

But while it would be quite untrue to say that this routine is never departed from, and equally profitless to attempt to find a point beyond which the bird will under no circumstances wander, yet there is enough definition and more than enough to answer the purpose for which the territory has, I believe, been evolved, that is to say the biological end of reproduction. Again, however, the process of adjustment is a complex one. Habit plays its part in determining the boundaries in a rough and ready manner, but the congenital basis, which is to be found in the behaviour adapted to a particular environment, is an important factor in the situation. For example, if instead of resting content with just a bare position sufficient for the purpose of reproduction, the Guillemot were to hustle its neighbours from adjoining ledges, the Guillemot as a species would probably disappear; or if instead of securing an area capable of supplying sufficient food both for itself and its young, the Chiffchaff were to confine itself to a single tree, and, after the manner of the Guillemot, trust to spasmodic excursions into neutral ground for the purpose of obtaining food, the Chiffchaff as a species would probably not endure. All such adjustments have, however, been brought about by relationships which have gradually become interwoven in the tissue of the race.

The intolerance that the male displays towards other individuals, usually of the same sex, leads to a vast amount of strife. Nowhere in the animal world are conflicts more frequent, more prolonged, and more determined than in the sexual life of birds; and though they are acknowledged to be an important factor in the life of the individual, yet there is much difference of opinion as to the exact position they occupy in the drama of bird life. Partly because they frequently happen to be in evidence, partly because they are numerically inferior, and partly, I suppose, because the competition thus created would be a means of maintaining efficiency, the females, by common consent, are supposed to supply the condition under which the pugnacious nature of the male is rendered susceptible to appropriate stimulation. And so long as the evidence seemed to show that battles were confined to the male sex, so long were there grounds for hoping that their origin might be traced to such competition. But female fights with female, pair with pair, and, which is still more remarkable, a pair will attack a single male or a single female; moreover, males that reach their destination in advance of their prospective mates engage in serious warfare. How then is it possible to look upon the individuals of one sex as directly responsible for the strife amongst those of the other, or how can the female supply the necessary condition? As long as an attempt is made to explain it in terms of the female, the fighting will appear to be of a confused order; regard it, however, as part of a larger process which demands, amongst other essential conditions of the breeding situation, the occupation of a definite territory, and order will reign in place of confusion.

But even supposing that the male inherits a disposition to acquire a suitable area, even supposing that it inherits a disposition which results indirectly in the defence of that area, how does it obtain a mate? If the female behaved in a like manner, if she, too, were to isolate herself and remain in one place definitely, that would only add to the difficulties of mutual discovery. We find, however, in the migrants, that the males are earlier than the females in reaching the breeding grounds, and, in resident species, that they desert the females and retire alone to their prospective territories, so that there is a difference in the behaviour of the sexes at the very commencement of the sexual process. What is the immediate consequence? Since the male isolates itself, it follows, if the union of the sexes is to be effected, that the discovery of a mate must rest largely with the female. This of course reverses the accepted course of procedure. But after all, what reason is there to suppose that, the male seeks the female, or that a mutual search takes place; what reason to think that this part of the process is subject to no control except such as may be supplied by the laws of chance?

Now, clearly, much will depend upon the rapidity with which the female can discover a male fit to breed; for if the course of reproduction is to flow smoothly, there must be neither undue delay nor waste of energy incurred in the search—some guidance is therefore necessary, some control in her external environment. Here the song, or the mechanically produced sound, comes into play, and assists in the attainment of this end. Nevertheless if every male were to make use of its powers whether it were in occupation of a territory or not, if the wandering individual had an equal chance of attracting a mate, then it would be idle to attempt to establish any relation between "song" on the one hand, and "territory" on the other, and impossible to regard the voice as the medium through which an effectual union of the sexes is procured. But there is reason to believe that the males utilise their powers of producing sound only under certain well-defined conditions. For instance, when they are on their way to the breeding grounds, or moving from locality to locality in search of isolation, or when they desert their territories temporarily, as certain of the residents often do, they are generally silent; but when they are in occupation of their territories they become vociferous—and this is notoriously the case during the early hours of the day, which is the period of maximum activity so far as sexual behaviour is concerned. So that just at the moment when the sexual impulse of the female is most susceptible to stimulation, the males are betraying their positions and are thus a guide to her movements. Nevertheless, even though she may have discovered a male ready to breed, success is not necessarily assured to her; for with multitudes of individuals striving to procreate their kind, it would be surprising if there were no clashing of interests, if no two females were ever to meet in the same occupied territory. Competition of this kind is not uncommon, and the final appeal is to the law of battle, just as an appeal to physical strength sometimes decides the question of the initial ownership of a territory.

I shall try to make clear the relations of the various parts to the whole with the assistance of whatever facts I can command. I shall do so not only for the purposes of the theory, but because one so often finds the more important features of sexual behaviour regarded as so many distinct phenomena requiring separate treatment, whereas they are mutually dependent, and follow one another in ordered sequence. I spoke of the process as a series of relationships. Some of these relationships have already been touched upon; others will become apparent if we consider for a moment the purposes for which the territory has been evolved. Indirectly its purpose is that of the whole process, the rearing of offspring. But inasmuch as a certain measure of success could be attained, and that perhaps often, without all the complications introduced by the territory, there are manifestly advantages to be gained by its inclusion in the scheme. The difficulties which beset the path of reproduction are by no means always the same—all manner of adjustments have to be made to suit the needs of different species. There are direct relationships, such as we have been speaking of, which are essential to the every-day working of the process, and others which are indirect, though none the less important for they must have exercised an influence throughout the ages. These latter are furnished by the physical—the inorganic world, by climate, by the supply of the particular kind of breeding stations, by the scarcity or abundance of the necessary food and by the relative position of the food supply to the places suitable for breeding. Why does the Reed-Bunting cling so tenaciously to an acre or more of marshy ground, while the Guillemot rests content with a few square feet on a particular ledge of rock? The answer is the same in both cases—to facilitate reproduction. But why should a small bird require so many square yards, whilst a very much larger one is satisfied with so small an area? The explanation must be sought in the conditions of existence. The Reed-Bunting has no difficulty in finding a position suitable for the construction of its nest; there are acres of waste land and reedy swamps capable of supplying food for large numbers of individuals, and the necessary situations for countless nests. But its young, like those of many another species, are born in a very helpless state. For all practical purposes they are without covering of any description and consequently require protection from the elements, warmth from the body of the brooding bird, and repeated supplies of nourishment. A threefold burden is thus imposed upon the parents: they must find food for themselves, they must afford protection to the young by brooding, and they must supply them with the necessary food at regular intervals. And their ability to do all this that is demanded of them will be severely taxed by the brooding which must perforce curtail the time available for the collection of food.

Let us then suppose that the Reed-Buntings inhabiting a certain piece of marsh are divided into two classes, those which are pugnacious and intolerant of the approach of strangers, and those which welcome their presence. The nests of the former will be built in isolation, those of the latter in close proximity. In due course eggs will be laid and incubation performed, and thus far all alike will probably be successful. Here, however, a critical point is reached. If the young are to be freed from the risk of exposure, the parents must find the necessary supply of food rapidly. But manifestly all will not be in a like satisfactory position to accomplish this, for whereas the isolated pairs will have free access to all the food in the immediate vicinity of the nest, those which have built in proximity to one another, meeting competition in every direction, will be compelled to roam farther afield and waste much valuable time by doing so; and under conditions which can well be imagined, even this slight loss of time will be sufficient to impede the growth of the delicate offspring, or to lead perhaps to still greater disaster. If any one doubts this, let him first examine one of the fragile offspring; let him then study the conditions under which it is reared, observing the proportion of time it passes in sleep and the anxiety of the parent bird to brood; and finally let him picture to himself its plight in a wet season if, in order to collect the necessary food, the parents were obliged to absent themselves for periods of long duration.

Now take the case of the Guillemot. Its young at birth are by no means helpless in the sense that the young Reed-Bunting is, and food is readily procured. But breeding stations are scarce, for although there are many miles of cliff-bound coast, yet not every type of rock formation produces the fissures and ledges upon which the bird rests. Hence vast stretches of coast-line remain uninhabited, and the birds are forced to concentrate at certain points, where year after year they assemble in countless numbers from distant parts of the ocean. If, then, different individuals were to jostle one another from adjoining positions, and each one were to attempt to occupy a ledge in solitary State, not only would the successful ones gain no advantage from the additional space over which they exercised dominion, but inasmuch as many members that were fitted to breed would be precluded from doing so, the status of the species as a whole would be seriously affected. The amount of space occupied by each individual is therefore a matter of urgent importance. A few square feet of rock sufficient for the immediate purpose of incubation is all that can be allowed if the species is to maintain its position in the struggle for existence.

Our difficulty in estimating the importance of the various factors that make for success or failure arises from our inability to see more than a small part of the scene as it slowly unfolds itself. The peculiar circumstances under which these cliff-breeding forms dwell does, however, enable us to picture, on the one hand, the precarious situation of an individual that was incapable of winning or holding a position at the accustomed breeding station, and, on the other, the plight of the species as a whole if each one exercised authority over too large an area. With the majority of species it is difficult to do this. So many square miles of suitable breeding ground are inhabited by so few Reed-Buntings that, even supposing certain members were to establish an ascendency over too wide an area, it would be impossible to discover by actual observation whether the race as a whole were being adversely affected. Competition doubtless varies at different periods and in different districts according to the numerical standing of the species in a given locality and according to the numerical standing of others that require similar conditions of existence; at times it may even be absent, just as at any moment it may become acute. These examples show how profoundly the evolution of the breeding territory may have been influenced by relationships in the inorganic world, and they give some idea of the intricate nature of the problem with which we have to deal.

I mentioned that the first visible manifestation of the revival of the sexual instinct was to be found in the movements undertaken by the males at the commencement of the breeding season. Such movements are characterised by a definiteness of purpose, whether they involve a protracted journey of some hundreds of miles or merely embrace a parish or so in extent, and that purpose is the acquirement of a territory suitable for rearing offspring. They are thus directly related to the territory, and the question arises as to whether their origin may not be traced to such relatedness. So long as we fix our attention solely upon the magnitude of the distance traversed the suggestion may seem a fanciful one. Nevertheless, if the battles between males of the same species are directly related to the occupation of a position suitable for breeding purposes, if those which occur between males of closely related forms can be traced to a similar source, if the females take their share in the defence of the ground that is occupied, if, in short, the competition is as severe as I believe it to be, and is wholly responsible for the strife which is prevalent at the commencement of the breeding season—then such competition must have introduced profound modifications in the distribution of species; it must have even influenced the question of the survival of certain forms and the elimination of others; and since the powers of locomotion of a bird are so highly developed it must have led to an extension of breeding range, limited only by unfavourable conditions of existence.

CHAPTER II

THE DISPOSITION TO SECURE A TERRITORY

Those who have studied bird life throughout the year are aware that the distribution of individuals changes with the changing seasons. During autumn and winter, food is not so plentiful and can only be found in certain places, and so, partly by force of circumstances and partly on account of the gregarious instinct which then comes into functional activity, different individuals are drawn together and form flocks of greater or less dimensions, which come and go according to the prevailing climatic conditions. But with the advent of spring a change comes over the scene: flocks disperse, family parties break up, summer migrants begin to arrive, and the hedgerows and plantations are suddenly quickened into life. The silence of the winter is broken by an outburst of song from the throats of many different species, and individuals appear in their old haunts and vie with one another in advertising their presence by the aid of whatever vocal powers they happen to possess—the Woodpecker utters its monotonous call from the accustomed oak; the Missel-Thrush, perched upon the topmost branches of the elm, persistently repeats its few wild notes; and the Swallow returns to the barn.

All of this we observe each season, and our thoughts probably travel to the delicate piece of architecture in the undergrowth, or to the hole excavated with such skill in the tree trunk; to the beautifully shaped eggs; to the parent birds carrying out their work with devoted zeal—in fact, to the whole series of events which complete the sexual life of the individual; and the attachment of a particular bird to a particular spot is readily accounted for in terms of one or other of the emotions which centre round the human home.

But if this behaviour is to be understood aright; if, that is to say, the exact position it occupies in the drama of bird life is to be properly determined, and its biological significance estimated at its true value, it is above all things necessary to refrain from appealing to any one of the emotions which we are accustomed to associate with ourselves, unless our ground for doing so is more than ordinarily secure. I shall try to show that, in the case of many species, the male inherits a disposition to secure a territory; or, inasmuch as the word "secure" carries with it too much prospective meaning, a disposition to remain in a particular place when the appropriate time arrives.

If the part which the breeding territory plays in the sexual life of birds is the important one I believe it to be, it follows that the necessary physiological condition must arise at an early stage in the cycle of events which follow one another in ordered sequence and make towards the goal of reproduction, and that the behaviour to which it leads must be one of the earliest visible manifestations of the seasonal development of the sexual instinct. When does this seasonal development occur? For how long does the instinct lie dormant? In some species there is evidence of this first step in the process of reproduction early in February; there is reason to believe that in others the latter part of January is the period of revival; and the possibility must not be overlooked of still earlier awakenings, marked with little definiteness, though nevertheless of sufficient strength to call into functional activity the primary impulse in the sexual cycle. Here, then, we meet with a difficulty so far as direct observation is concerned, for the duration of the period of dormancy and the precise date of revival vary in different species; and, if accurate information is to be obtained, the study of the series of events which culminate in the attainment of reproduction ought certainly to begin the moment behaviour is influenced by the internal changes, whatever they may be, which are responsible for the awakening of the sexual instinct.

In considering how this difficulty might be met, the importance of migratory species as a channel of information was gradually borne in upon me; for it seemed that the definiteness with which the initial stage in the sexual process was marked off, as a result of the incidence of migration, would go far towards removing much of the obscurity which appeared to surround the earlier stages of the breeding problem in the case of resident species. Recent observation has shown that I exaggerated this difficulty, and that it is generally possible to determine with reasonable accuracy the approximate date at which the internal changes begin to exert an influence on the behaviour of resident species also. Nevertheless, the specialised behaviour of the migrants furnished a clue, and pointed out the direction which further inquiry ought to take.

Those who are accustomed to notice the arrival of the migrants are aware that the woods, thickets, and marshes do not suddenly become occupied by large numbers of individuals, but that the process of "filling up" is a gradual one. An individual appears here, another there; then after a pause there is a further addition, and so on with increasing volume until the tide reaches its maximum, then activity wanes, and the slowly decreasing number of fresh arrivals passes unnoticed in the wealth of new life that everywhere forces itself upon our attention. If now, instead of surveying the migrants as a whole, our attention be directed to one species only, this gradual arrival of single individuals in their accustomed haunts will become even more apparent; and if the investigation be pursued still further and these single individuals observed more closely, it will be found that in nearly every case they belong to the male sex. Males therefore arrive before females. This does not mean, however, that the respective times of arrival of the males and females belonging to any one species are definitely divided, for males continue to arrive even after some of the females have reached their destination; and thus a certain amount of overlapping occurs. A truer definition of the order of migration would be as follows:—Some males arrive before others, and some females arrive before others, but on the average males arrive before females. This fact has long been known. Gätke refers to it in his Birds of Heligoland. "Here in Heligoland," he says, "the forerunners of the spring migration are invariably old males; a week or two later, solitary old females make their appearance; and after several weeks, both sexes occur mixed, i.e., females and younger males; while finally only young birds of the previous year are met with." Newton alludes to it as follows:—"It has been ascertained by repeated observation that in the spring movement of most species of the northern hemisphere, the cock birds are always in the van of the advancing army, and that they appear some days, or perhaps weeks, before the hens"; and Dr Eagle Clarke, in his Studies in Bird Migration, makes the following statement:—"Another characteristic of the spring is that the males, the more ardent suitors, of most species, travel in advance of the females, and arrive at their meeting quarters some days, it is said in some cases even weeks, before their consorts." Some interesting details were given in British Birds[1] in regard to the sex of the migrants that were killed by striking the lantern at the Tuskar Rock, Co. Wexford, on the 30th April 1914. In all, there were twenty-four Whitethroats, nine Willow-Warblers, eight Sedge-Warblers, and six Wheatears; and on dissection it was found that twenty Whitethroats, seven Willow-Warblers, eight Sedge-Warblers, and one Wheatear were males.

What a curious departure this seems from the usual custom in the animal world! Here we have the spectacle afforded us of the males, in whom presumably the sexual instinct has awakened, deserting the females just at the moment when we might reasonably expect their impulse to accompany them would be strongest; and this because of their inherited disposition to reach the breeding grounds. If, in order to attain to reproduction, the male depended primarily upon securing a female—whether by winning or fighting matters not at the moment—if her possession constituted the sole difference in his external environment between success and failure, then surely one would suppose that an advantage must rest with those individuals which, instead of rushing forward and inflicting upon themselves a life of temporary isolation, remained with the females and increased their opportunities for developing that mutual appreciation which, by some, is held to be a necessary prelude to the completion of the sexual act, and to which close companionship would tend to impart a stimulus.

In thus speaking, however, we assume that the revival of the sexual instinct in the migratory male is coincident in time with its return to the breeding quarters; and we do so because the act of migrating is believed to be the first step in the breeding process. But it is well to bear in mind just how much of this assumption is based upon fact, and how much is due to questionable inference. All that can be definitely asserted is this, that appropriate dissection reveals in most of the migrants, upon arrival at their destination, unquestionable evidence of seasonal increase in the size of the sexual organs. Beyond this there is nothing to go upon. Yet if the term "sexual instinct" is held to comprise the whole series of complex relationships which are manifest to us in numerous and specialised modes of behaviour, which ultimately lead to reproduction, and which have gradually become interwoven in the tissue of the race, there can be little doubt that the assumption is a reasonable one. To some, the term may recall the fierce conflicts which are characteristic of the season; to others, emotional response; to not a few, perhaps, the actual discharge of the sexual function—all of these, it is true, are different aspects of the one instinct; but at the same time each one marks a stage in the process, and the different stages follow one another in ordered sequence. However, we are not concerned at the moment with the term in its wider application; we wish to know the precise stage at which the disposition to mate influences the behaviour of the male. Is the female to him, from the moment the seasonal change in his sexual organs takes place, a goal that at all costs must be attained? Or is it only when the cycle of events which leads up to reproduction is nearing completion that she looms upon his horizon? One would like to be in a position to answer these questions, but there is nothing in the way of experimental evidence to go upon; and if I say that there is reason to believe that, in the earlier stages, the female is but a shadow in the external environment of the male, it must be taken merely as an expression of opinion, though based in some measure upon a general observation of the behaviour of various species.

Before attempting to explain the difference in the times of arrival of the male and female migrant, let us examine the behaviour of some resident species at a corresponding period. My investigations have been made principally amongst the smaller species—the Finches and the Buntings—which often pass the winter in or near the localities wherein they brought up offspring or were reared. It is true that they wander from one field to another according to the abundance or scarcity of food; it is also true that, if the weather is of a type which precludes the possibility of finding the necessary food, these wanderings may become extensive or even develop into partial migrations. But under the normal climatic conditions which prevail in many parts of Britain, these smaller resident species seem to find all that they require without travelling any great distance from their breeding haunts. Flocks composed of Yellow Buntings, Cirl Buntings, Corn-Buntings, Chaffinches, Greenfinches, etc., can be observed round the farmsteads or upon arable land; small flocks of Reed-Buntings take up their abode on pieces of waste land and remain there until the supply of food is exhausted, deserting their feeding ground only towards evening when they retire to the nearest reed-bed to pass the night; flocks of Hawfinches visit the same holly-trees day after day so long as there is an abundance of berries on the ground beneath; and so on.

I have mentioned the Reed-Bunting; let us take it as our first example and try to follow its movements when the influence exerted by the internal secretions begins to be reflected on the course of its behaviour. First, it will be necessary to discover the exact localities in any given district to which the species habitually returns for the purpose of procreation; otherwise the earlier symptoms of any disposition to secure a territory may quite possibly be overlooked in the search for its breeding haunts.

In open weather Reed-Buntings pass the winter either singly, in twos or threes, or in small flocks, on bare arable ground, upon seed fields, or in the vicinity of water-courses; but in the breeding season they resort to marshy ground where the Juncus communis grows in abundance, to the dense masses of the common reed (Arundo phragmites), and such like places. During the winter, the male's routine of existence is of a somewhat monotonous order, limited to the necessary search for food during the few short hours of daylight and enforced inactivity during the longer hours of darkness. But towards the middle of February a distinct change manifests itself in the bird's behaviour. Observe what then happens. When they leave the reed-bed in the morning, instead of flying with their companions to the accustomed feeding grounds, the males isolate themselves and scatter in different directions. The purpose of their behaviour is not, however, to find fresh feeding grounds, nor even to search for food as they have been wont to do, but rather to discover stations suitable for the purpose of breeding; and, having done so, each male behaves in a like manner—it selects some willow, alder, or prominent reed, and, perching thereon, leads a quiet life, singing or preening its feathers. Now if the movements of one particular male are kept in view, it will be noticed that only part of its time is spent in its territory. At intervals it disappears. I do not mean that one merely loses sight of it, but that it actually deserts its territory. As if seized with a sudden impulse it rises into the air and flies away, often for a considerable distance and often in the same direction, and is absent for a period which may vary in length from a few minutes to an hour or even more. But these periodical desertions become progressively less and less frequent in occurrence until the whole of its life is spent in the few acres in which it has established itself.

The behaviour of the Yellow Bunting is similar. In any roadside hedge two or more males can generally be found within a short distance of one another, and in such a place their movements can be closely and conveniently followed. Under normal conditions the ordinary winter routine continues until early in February; but the male then deserts the flock, seeks a position of its own, and becomes isolated from its companions. Now the position which it selects does not, as a rule, embrace a very large area—a few acres perhaps at the most. But there is always some one point which is singled out and resorted to with marked frequency—a tree, a bush, a gate-post, a railing, anything in fact which can form a convenient perch, and eventually it becomes a central part of the bird's environment. Here it spends the greater part of its time, here it utters its song persistently, and here it keeps watch upon intruders. The process of establishment is nevertheless a gradual one. The male does not appear in its few acres suddenly and remain there permanently as does the migrant; at first it may not even roost in the prospective territory. The course of procedure is somewhat as follows:—At dawn it arrives and for a while utters its song, preens its feathers, or searches for food; then it vanishes, rising into the air and flying in one fixed direction as far as the eye can follow, until it becomes a speck upon the horizon and is ultimately lost to view. During these excursions it rejoins the small composite flocks which still frequent the fields and farm buildings. For a time the hedgerow is deserted and the bird remains with its companions. But one does not have to wait long for the return; it reappears as suddenly as it vanished, flying straight back to the few acres which constitute its territory, back even to the same gate-post or railing, where it again sings. This simple routine may be repeated quite a number of times during the first two hours or so of daylight, with, of course, a certain amount of variation; on one occasion the bird may be away for a few minutes only, on another for perhaps half an hour, whilst sometimes it will fly for a few hundred yards, hesitate, and then return—all of which shows clearly enough that these few acres possess some peculiar significance and are capable of exercising a powerful influence upon the course of its behaviour. And so the disposition in relation to the territory becomes dominant in the life of the bird.

Or take the case of the Chaffinch. In winter large or small flocks can be found in many varied situations. But in the latter part of February, or the early days of March, these flocks begin to disperse. At daylight males can then be observed in all kinds of situations, either calling loudly, uttering their spring note, or exercising their vocal powers to the full; and it will be found that, in the majority of instances, these males are solitary individuals, that they pass the early hours of the morning alone, and that their normal routine of calling, singing, or searching for food, is only interrupted by quarrels with their neighbours. The same locality is visited regularly—not only the same acre or so of ground, but even the same elm or oak, has, as its daily occupant, the same cock Chaffinch. And temporary desertions from the territory occur also, much like those referred to in the life of the Bunting, but perhaps not so frequently. One has grown so accustomed during the dark days of winter to the sociable side of Chaffinch behaviour—to the large flocks searching for food, to the endless stream of individuals returning in the evening to roost in the holly-trees, to the absence of song—that this radical departure from the normal routine comes as something of a surprise; for the days are still short, the temperature is still low, the nesting season is still many weeks ahead, and yet for part of the day, and for just that part when the promptings of hunger must be strongest, the male, instead of joining the flock, isolates itself and expends a good deal of energy in insuring that its isolation shall be complete. And in place of the silence we hear from all directions the cheerful song uttered with such marked persistency that it almost seems as if the bird itself must be aware that by doing so it was advertising the fact of its occupation of a territory. This is surely a remarkable change, and the females in the meantime continue their winter routine.

One other example. The monotonous call of the Greenfinch is probably familiar to all. In winter these birds accompany other Finches and form with them flocks of varying sizes, but in the spring the flocks disperse, and the Greenfinch, in common with other units of the flock, alters its mode of life. But whereas the Chaffinch or the Bunting begins to acquire its territory in February, the Greenfinch only does so in April. When the organic changes do at length begin to make themselves felt, the male seeks a position of its own, and having found one remains there, uttering its characteristic call. But owing probably to the fact that it is much later than the aforementioned species in acquiring a territory, temporary desertions are not so much in evidence. The species is so very plentiful, and the bird is so prone to nest in gardens and shrubberies surrounding human habitations, that this seasonal change in its routine of existence cannot fail to be noticed. One can hear its call in every direction, one can watch the same individual in the same tree; and it is the male that is thus seen and heard, the female appears later. Thus the behaviour falls into line with that of the Bunting or the Chaffinch.

The behaviour of these resident species throws some light upon the early arrival of the males which we are endeavouring to explain in the case of the migrants. Let us see how their actions compare. The male resident deserts the female early in the year and establishes itself in a definite position, where it advertises its presence by song; the male migrant travels from a great distance, arrives later, and also establishes itself in a definite position, where it, too, advertises its presence by song. The male resident passes only the earlier part of the day in its territory at the commencement of the period of occupation; the male migrant remains there continuously from the moment it arrives. The male resident deserts its territory at intervals, even in the morning; the male migrant betrays no inclination to do so. Thus there is a very close correspondence between the behaviour of the two, and what difference there is—slight after all—cannot be said to affect the main biological end of securing territory. One is apt to think of the problem of migration in terms of the species instead of in terms of the individual. One pictures a vast army of birds travelling each spring over many miles of sea and land, and finally establishing themselves in different quarters of the globe; and so it comes about, I suppose, that a country or some well-defined but extensive area is regarded as the destination, the ultimate goal, of the wanderers. But the resident male has a journey to perform, short though it may be; it, too, has a destination to reach, neither a country nor a locality, but a place wherein the rearing of offspring can be safely accomplished, and it, too, arrives in that place in advance of the female.

With these facts at our disposal, we will endeavour to find an explanation. It is unlikely that specialised behaviour would occur in generation after generation under such widely divergent conditions, and, moreover, expose the birds to risk of special dangers, if it were but an hereditary peculiarity to which no meaning could be attached. Hence the appearance of the males in their breeding haunts ahead of the females becomes a fact of some importance, and suggests that the extensive journey in the one case, and the short journey in the other, may both have a similar biological end to serve.

Darwin evidently attached importance to this difference between the males and the females in their times of arrival. In the Descent of Man he referred to it as follows: "Those males which annually first migrated in any country, or which in spring were first ready to breed, or were the most eager, would leave the largest number of offspring; and these would tend to inherit similar instincts and constitutions. It must be borne in mind that it would have been impossible to change very materially the time of sexual maturity in the females without at the same time interfering with the period of the production of the young—a period which must be determined by the season of the year." Newton suggested the following explanation[2]: "It is not difficult to imagine that, in the course of a journey prolonged through some 50° or 60° of latitude, the stronger individuals should outstrip the weaker by a very perceptible distance, and it can hardly be doubted that in most species the males are stouter, as they are bigger than the females." Granting that the males are the stronger, how can this account for their outstripping the females by a week, ten days, or even a fortnight, in a journey of perhaps 1500 miles? To expect the birds to accomplish such a distance in seven days is surely not estimating their capabilities too highly, and any slight inequality in the power of flight or endurance could give the males an advantage of a few hours only. But this explanation, based upon inequalities in the power of flight and endurance on the one hand, and the magnitude of the distance traversed on the other, cannot afford a solution of the behaviour of the resident males, and is less likely, therefore, to be a true solution of that of the migrants.

There is another theory, simple enough in its way, which will probably occur to many. It is based on the assumption that the males reach sexual maturity before the females; and it is contended that the functioning of the instincts which contribute towards the biological end of reproduction depend upon the organic changes which the term "sexual maturity" is held to embrace, and that, inasmuch as the migratory instinct belongs to the group of such instincts, the males must be the first to leave their winter quarters.

What is meant by the "migratory instinct"? To speak of it as one of the instincts concerned in reproduction is not enough. Reproduction involves the actual discharge of the sexual function, which involves the females; but the first visible manifestation of organic change in the male is its desertion of the females. Yet this is the behaviour which is referred to as the "migratory instinct," and which comes into play, according to this theory, because the bird has reached sexual maturity. Manifestly we must have some clear understanding as to what these terms represent. That organic changes determine the functioning of certain definite instincts at certain specified times there can be no doubt; that these changes may occur at a somewhat earlier date in the male than in the female is more than probable, but that this explains the behaviour in question I do not believe. One wants to know why the changes should occur earlier in the male, what disposition it is which first comes into functional activity, and to what such disposition is related.

It may, however, be urged that, after all, this apparent eagerness to reach the breeding grounds is but a modification of hereditary procedure under the guiding hand of experience. What more likely result would follow from the enjoyment associated with previous success in the attainment of reproduction than a craving to repeat the experience? What stronger incentive to a hurried return could be imagined? It must be admitted that there are certain facts which might be used in support of an appeal to experience as a reasonable explanation. For example, the first males to arrive often display that richness of colouring which is generally supposed to indicate a fuller maturity. Gätke even speaks of the "most handsome old birds being invariably the first to hasten back to their old homes." But if experience is a factor, if some dim recollection of the past is held to explain the hurried departure of the male migrant, one wants to know with what such recollection is associated. Is it associated with the former female, or with the former breeding place, or with both? I take it that any recollection, no matter how vague, must be primarily associated with the particular place wherein reproduction had previously been accomplished; and I grant that if the first individuals to appear were invariably the older and experienced birds, their early return might be explained on the basis of such an association. But if there is reason to believe that a proportion are young birds on the verge of carrying out their instinctive routine for the first time, then we cannot appeal to past experience in explanation of their behaviour.

The age of a bird is difficult to determine. Experience leads me to believe that some of the males that arrive before the females are birds born the previous season; one finds, for instance, individuals with plumage of a duller hue, which denotes immaturity, amongst the first batch of arrivals. But though plumage may sometimes be a satisfactory guide, yet to rely upon it alone, or upon a more perfect development of feather, is to exceed the limits of safety. How, then, can we ascertain whether all the males that arrive before the females have had some previous experience of reproduction? Well, we take a particular locality and note the migrants that visit it year after year, and we find that the respective numbers of the different species are subject to wide annual fluctuations. Not every species lends itself to an inquiry of this kind: some are always plentiful and fluctuation is consequently difficult to discern; others are scarce and variation is easily determined. Those which are of local distribution but conspicuous by their plumage, or easily traced by the beauty or the peculiarity of their song, afford the more suitable subjects for investigation. For example, the Grasshopper-Warbler, Marsh-Warbler, Nightingale, Corncrake, Red-backed Shrike, or Whinchat have each some distinctive peculiarity which makes them conspicuous, and each one is subject to marked fluctuation in numbers. The small plantation or wooded bank may hold a Nightingale one year, but we miss its song there the next; the osier bed or gorse-covered common which vibrates with the trill of the Grasshopper-Warbler one April is deserted the following season; the plantation which is occupied by a host of common migrants this summer may be enlivened next year by the song of the rarer Marsh-Warbler also; and so on. The fluctuation is considerable: we observe desertion on the one hand, appropriation on the other, and yet males appear before females whether the particular plantation, osier bed, or swamp had been inhabited or not the previous season. This fact is not without significance. It shows that similar conditions prevail both amongst the males that appropriate breeding grounds new to them, and amongst those that return to some well-established haunt; and on the assumption that the earlier arrivals are experienced males, the same birds evidently do not return to the same place year after year. Granting, then, that the males which appropriate new breeding-grounds are young birds, how can their earlier arrival be explained in terms of past experience; and granting that they are old, and therefore experienced, how can it be explained in terms of association?

Again, it may be urged that if there is some biological end to be furthered by this hurried return, and if recollection of past experience is a means towards that end, such recollection need not necessarily be associated with a definite place, but only in a vague way with the whole series of events leading up to reproduction—in which series the migratory journey may even have acquired meaning. Whether there be any recollection of a previous journey or of a nest with young, I do not know. But the young bird is capable of performing its journey, of building its nest, and of rearing its young antecedent to experience—racial preparation has fitted it thus far; why then exclude the other event in the series, the earlier departure of the male, from hereditary equipment? If the journey were a casual affair without any goal attaching to it, if the males upon arrival wandered about in search of a mate, there would be some ground for thinking that a vague recollection of the whole former experience was sufficient to explain the hurried return; but since the pleasurable effect of association, founded upon previous experience of a definite place, cannot well be established, and since it is so difficult to study the objective aspect of the behaviour in question without coming to the conclusion that the journey is related to the appropriation of a place suitable for the rearing of offspring, one is tempted to ask whether the hurried return may not also be so related.

Now the males of some of the migratory species, especially of those which are accustomed to return to their breeding haunts early in the season, are called upon to face greater dangers and have a greater strain imposed upon their strength by starting forth upon their journey ten days or a fortnight before their prospective mates. The blizzards which so often sweep across the northern parts of Europe in the latter half of March, destroying in their course the all too scanty supply of insect life, may take toll of their numbers; or the westerly gales, which are not infrequent at that period, may meet them in mid-ocean and add to the perils of their journey; or the temperature of the previous weeks may have been sufficiently low to arrest the development of insect life—and yet males are annually exposed to these risks in hurrying to their breeding grounds. For what purpose? The answer will largely depend upon the way in which we regard those few acres wherein a resting place is ultimately found. For myself, I believe that they are of importance, inasmuch as the securing of a place suitable for the rearing of offspring is a primary condition of success in the attainment of reproduction; and if this be so, it is evident that the interests of the race will be better served by the males making good this first step before the females are ready to pair, otherwise they might oscillate between two modes of behaviour, created by the premature functioning of conflicting impulses.

The different steps in the process seem to follow one another in ordered sequence. The male inherits a disposition—which for us, of course, has prospective meaning—to seek the appropriate breeding ground and there to establish itself; and as early a functioning of this disposition as possible, consonant with the conditions of existence in the external environment, may have been evolved for the following reasons—firstly, the earlier individuals will meet with less interference wherever they may settle, every locality will be open to them, every acre free, their only need being that particular environment for which racial preparation has fitted them. In the second place, being already established when other males appear upon the scene, and advertising their presence by song, they will be less liable to molestation; thirdly, in those cases in which a long journey is undertaken, they will have ample time to recover from the fatigue, and, if attacked by later arrivals, will thus be in a better position to defend their territories; and lastly, a greater uniformity in their distribution will be insured before the females begin their search.

There is, besides, another good reason for thinking that the earlier males will have an advantage. We will assume—and from the abundant evidence supplied by the marking of birds, it is quite a reasonable assumption—that there is a tendency, generally speaking, for individuals to return to the neighbourhood of their birthplace, or to the place in which they had previously reared their offspring. Now the earlier arrivals will have no difficulty in securing territories; those that come later may have to search more diligently, still they will gain all that they require so long as any available space remains. Then comes the point when all suitable ground is occupied, and yet there are males to be provided for. What will be the position of these males? Urged by their inherited nature, they will leave the district and possibly continue their search into those adjoining, only, however, to add to the difficulties of the males there similarly situated; and even allowing that they are at length successful in establishing themselves, what are their prospects of securing mates? Since the earlier females will not extend their wanderings farther than is absolutely necessary, but will pair whenever the opportunity for doing so arises, it is to the later females, forced onwards by competition, that the late males must look for mates; so that when at length pairing does take place, much valuable time will have been lost.

The disadvantages which the late arrivals have to face are therefore great, and it is probable that the percentage which attain to reproduction will on the average be somewhat lower than the percentage in the case of the earlier arrivals. The district in which my observations have been made lies well within the limits of the breeding range of most of our common species, and it is not surprising that I should have met with little evidence of failure to breed as a result of failure to secure territory. Some interesting information was supplied to me, however, by the late Robert Service. He found, in certain seasons in Dumfriesshire, flocks of from ten to fifty unmated Sedge-Warblers, which, from the time of their arrival in May until the middle of July, haunted reed-filled spaces along stagnant streams. These flocks appeared to him to be composed of loosely-attached individuals of a migrant flock that had failed to find things congenial enough to entice them to disperse. But may they not have been composed of males that had failed to secure territories, or of females that had failed to discover males in possession of territories, or of both?

We have seen that, in the case of many species, each male establishes itself in a particular place at the commencement of the breeding season, even though this may mean a partial or perhaps a complete severance from former companions. We must now discuss this fact in greater detail because it is opposed to the views often held regarding the sexual behaviour of birds, and is manifestly of importance when considering the theory of breeding territory.

First, however, there is a point which requires some explanation. I speak of the same male being in the same place. How can I prove its identity? In the first place it is highly improbable that a bird which roams about within the same small area of ground, makes regular use of a certain tree and a certain branch of that tree, and observes a similar routine day after day, can be other than the same individual. But, apart from this general consideration, are there any means by which individuals of the same species can be identified? Well, there is variation in the plumage. Supposing we take a dozen cock Chaffinches and examine them carefully, we shall find slight differences in pattern and in colour—more grey here or a duller red there, as the case may be—and though these differences may not be sufficient to enable us to pick out a bird at a distance, they are nevertheless conspicuous when it is close at hand. Then again there is variation in the song; and the more highly developed the vocal powers the greater scope there is for variation. But even the phrases of a simple song can be split up and recombined in different ways. If one were asked casually whether the different phrases of the Reed-Bunting's song always followed one another in the same sequence, the answer would probably be that they certainly did so, whereas the bird is capable of combining the few notes it possesses in a surprising number of different ways. And lastly, there are differences in just the particular way in which specific behaviour, founded upon a congenital basis, is adapted by each individual to its own special environment. Racial preparation determines behaviour as a whole, but the individual is allowed some latitude in the execution of details which are in themselves of small moment—the selection of a particular tree as a headquarters and a particular branch upon that tree, the direction of the distant excursion, and the direction of the limited wanderings within the small area surrounding the headquarters which in the course of time determine the extent of the territory, are matters for each individual to decide when the occasion for doing so arises. Moreover instances of abnormal coloration or abnormal song are not rare, and they are valuable since they place the identity of the individual beyond dispute. I can recall the case of a Willow-Warbler whose song was unlike that of its own or any other species, and of a Redbreast whose voice puzzled me not a little. I can recollect also a male Yellow Bunting whose foot was injured or deformed. Of this bird's behaviour I kept a record for two months or so; and inasmuch as it inhabited a roadside hedge, and was of fearless disposition, the deformed foot could plainly be seen whenever it settled upon the road to search for food. Identification is not, therefore, a difficulty. There is always some small difference in colour or in song, or some well-defined routine which makes recognition possible.

Owing to their great powers of locomotion, birds have generally been regarded as wanderers more or less; anything in the nature of a fixed abode, apart from the actual nest, having been accounted foreign to their mode of life; and even the locality immediately surrounding the nest has not been apprehended as possessing any meaning for the owner of that nest. No doubt the supply of food determines their movements for a considerable part of the year; they seek it where they can find it, here to-day, there to-morrow—in fact few species fail to move their quarters at one season or another, so that there is much truth in the notion that birds are wanderers. Yet to suppose that every individual one sees or hears—every Lapwing on the meadow, or Nightingale in the withy bed—is in that particular spot just because it happens to alight there as it roams from place to place, is to take a view which the observed facts do not support. For as soon as the question of reproduction dominates the situation, a new condition arises, and the habits formed during the previous months are reversed, and the males, avoiding one another, or even becoming actively hostile, prefer a life of seclusion to their former gregariousness—all of which occurs just at the moment when we might reasonably expect them to exhibit an increased liveliness and restlessness as a result of their endeavour to secure mates; and so universal is the change that it might almost be described as an accompaniment of the sexual life of birds generally.

That the Raven and certain birds of prey exert an influence over the particular area which they inhabit has long been known, and it has been recognised more especially in the case of the Peregrine Falcon, possibly because the bird lives in a wild and attractive country, and, forcing itself under the notice of naturalists, has thus had a larger share of attention devoted to its habits. Moreover, when a species is represented by comparatively few individuals, and each pair occupies a comparatively large tract of country, it is a simple matter to trace the movements and analyse the behaviour of the birds. There is a rocky headland in the north-west of Co. Donegal comprising some seven miles or so of cliffs, where three pairs of Falcons and two pairs of Ravens have nested for many years. Each year the different pairs have been more or less successful in rearing their young; each year the young can be seen accompanying their parents up to the time when the sexual instinct arises; and yet the actual number of pairs is on the whole remarkably constant, and there is no perceptible increase. It seems as if the numbers of three and two respectively were the maximum the headland could maintain. But this is no exceptional case; it represents fairly the conditions which obtain as a rule amongst those species, granting, of course, a certain amount of variation in the size of each territory determined by the exigencies of diverse circumstances.

If we take a given district, and devote our attention to the smaller migrants that visit Western Europe each returning spring for the purpose of procreation, we shall find that the movements of the males are subject to a very definite routine. This, however, is not true of every male; some may be wending their way to breeding grounds at a distance; others may be seeking the particular environment to which they may be adapted; others again, having found their old haunts destroyed, may consequently be seeking new.

Of all this there is evidence. Small parties of Chiffchaffs pass through a district on their way to other breeding grounds, flitting from hedge to hedge as they move in a definite direction with apparently a definite purpose; Reed-Warblers settle in a garden or plantation, eminently unsuited to their requirements, and disappear; Wood-Warblers arrive in some old haunt, and finding it no longer suitable for their purpose, seek new ground. So that plenty of individuals are always to be found, which, for the time being at least, are wanderers.

In the district which I have in mind, the wandering males form only a small part of the incoming bird population. The majority of individuals that fall under observation are those that have made this particular district their destination; and in doing so, they may possibly have been guided by their experience as owners or inmates of former nests, for it cannot be doubted that a return to the neighbourhood of the birthplace would lead to a more uniform distribution and therefore be advantageous, and the tendency to do so might consequently have become interwoven in the tissue of the race. How, then, do they behave? A certain amount of movement, an interchanging of positions, even though restricted to an area defined, let us say, by experience, might be expected under the circumstances—that, however, is not what we find; we observe the available situations plotted out into so many territories, each one of which is occupied by a male who passes the whole of his time therein. Take whatever species we will—Whitethroat, Whinchat, Willow-Warbler, Red-backed Shrike, it matters not which, for there is no essential difference in the general course of procedure—this condition will be found to prevail. Generally speaking, the behaviour in relation to the territory can be studied more conveniently where a number of individuals of the same species have established themselves in proximity to one another. Such species as the Chiffchaff, Willow-Warbler, or Wood-Warbler are often sufficiently common to allow of three or more of their respective males being kept in view at the same time; and the disposition to occupy a definite position can be readily observed. The Reed-Warbler is a suitable subject for an investigation of this kind; for since it is restricted by its habits to localities wherein the common reed (Arundo phragmites) grows in abundance, and since such localities are none too plentiful and often limited in extent, the area occupied by each individual is necessarily small—if it were not so the species would become extinct. Hence it is a simple matter to study the routine of the different individuals and to mark the extent of their wanderings.

In this way the males of all the Warblers that breed commonly in Great Britain establish themselves, each one in its respective station at the respective breeding ground; so, too, do those of many other migrants—for example, the Whinchat, Wheatear, Tree-Pipit, and Red-backed Shrike. All of these, it is true, are common species—numbers of individuals can often be found in close proximity—and therefore it may be argued that they keep to one position more from pressure of population than from any inherited disposition working towards that end. But the rarer species behave similarly. Districts frequented by the Marsh-Warbler and offering plenty of situations of the type required by the bird are often inhabited by a few members only, and yet the disposition to remain in a definite position is just as marked.

You will say, however, that these smaller migrants have no exceptional powers of flight; that they have besides just completed a long and arduous journey; and you will ask why they should be expected to wander, whether it is not more reasonable to expect that, in order to overcome their fatigue, they should remain where they settle. The Cuckoo is a wanderer in the wider sense of the term, and is gifted with considerable powers of flight. Upon arrival the male flies briskly from field to field, showing but little signs of weariness; yet we have only to follow its movements for a few days in succession to assure ourselves that the bird is no longer a wanderer; for just as the Warbler or the Chat moves only within a definitely delimited area, so the male Cuckoo, strange as it may seem, restricts itself to a particular tract of land. The area over which it wanders is often considerable and consequently it is not possible to keep the bird always in view, but inasmuch as the variation in the voices of different individuals is quite appreciable, identification is really a simple matter. If we cannot keep the bird in sight, we can trace its movements by sound and mark the extent of its wanderings, which by repetition become more and more defined, until a belt of trees here, or an orchard there, mark a rough and rarely passed boundary line.

Let us take another example from the larger migrants—the Black-tailed Godwit, a bird common enough in the Dutch marshes but no longer breeding in this country. On suitable stretches of marsh land, numbers will be found in proximity one to another after the manner of the Lapwing, each male occupying a definite space of ground wherein it passes the time preening, searching for food, or in sleep—though at the same time keeping a strict watch over its territory. Now the preference shown for a particular piece of ground, and the determination with which it is resorted to, is the more remarkable when we take into consideration the specific emotional behaviour arising from the seasonal sexual condition. This behaviour is expressed in a peculiar flight. The bird rises high in the air, circles round with slowly beating wings above the marsh, and utters a call which, as far as my experience goes, is characteristic of the performance. The air is often full of individuals circling thus even beyond the confines of the marsh, for a male does not limit its flight to a space immediately above its territory; but nevertheless careful observation will show how unerringly each one returns to its own position on the breeding ground, no matter how extensive the aerial excursion may have been. And so, when the males of the smaller migrants confine their movements to an acre of ground at the completion of their long journey, they are acting no more under the influence of fatigue than the Cuckoo, which keeps within certain bounds yet flies about briskly, or the Godwit which, though holding to its few square yards on the ground, executes most tiring and extensive flights above the marsh.

Of all the migrants, however, the behaviour of the Ruff is perhaps the most strange, and though it has long been known that these birds have their special meeting places where they perform antics and engage in serious strife, yet it is only within recent years that the primary purpose of these gatherings has been ascertained—that purpose being the actual discharge of the sexual function. Mr. Edmund Selous has carried out some exhaustive investigations into their activities at the meeting places, and he makes it clear that each bird has its allotted position. He says, for example, that "It begins to look as though different birds had little seraglios of their own in different parts of the ground," that "each Ruff has certainly a place of its own," or again that "this Ruff indeed, which I think must be a tender-foot, does not seem to have a place of its own like the others." Nevertheless it is only at the meeting places that they have their special positions; there is no evidence to show that each one has a special territory, wherein it seeks its food, as the Warbler has, and therefore some may think that we are here confronted with behaviour of a different order. But we must bear in mind that the process has been adjusted to meet the requirements of different species: the size of the territory, the period of its daily occupation, the purpose which it serves—these all depend upon manifold relationships and do not affect the principle. Why it has been differentiated in different circumstances we shall have occasion to discuss later; for the moment it is enough that at the end of its migratory journey each Ruff occupies one position on the meeting ground.

Now birds that are paired for life, whose food-supply is not affected by alternations of climate, have no occasion to desert the locality wherein they have reared their offspring, and so their movements, being subject to a routine which would tend to become increasingly definite, must in the course of time and according to the law of habit formation become organised into the behaviour we observe. Is it necessary, therefore, to seek an explanation of their tendency to remain in one place in anything so complex as an inherited disposition? Again, since we have to confess to so very much ignorance on so many points connected with the whole phenomenon of migration, may there not be some condition, hitherto shrouded in mystery, which might place so different a complexion on the corresponding aspect of migrant behaviour as to rid us, in their case also, of the necessity of appealing to an inherited disposition? Such questions are justifiable. And if the life-histories of other species gave no further support to our interpretation, if, in short, the evidence were to break down at this point, then we should be forced to seek some other explanation more in keeping with the general body of facts.

But far from placing any obstacle in the way of an interpretation in terms of inherited disposition, the behaviour of many of those residents which are not paired for life gives us even surer ground for that belief. Moreover in their case the initial stages in the process are more accessible to observation. I will endeavour to explain why. In the process of reproduction the environment has its part to play—whether in the manner here suggested, or indirectly through the question of food-supply, matters not at the moment. Now, migratory species are more highly specialised than resident species as regards food, and are affected more by variations of temperature, so that they can live for only a part of the year in the countries which they visit for the purpose of procreation. Hence the organic changes, which set the whole process in motion, must be coincident in time with the growth of appropriate conditions in the environment; for if it were not so, if the internal organic changes were to develop prematurely, the bird would undertake its journey only to find an insufficiency of food upon its arrival, and this would scarcely contribute towards survival. Definite limitations have therefore been imposed upon the period of organic change. But in the case of many resident species the conditions are somewhat different, for they remain in the same locality throughout the year, and a gradual unfolding of the reproductive process cannot therefore have a similarly harmful effect. Thus it comes about that the behaviour of the migrant, when it arrives at the breeding ground and first falls under observation, represents a stage in the process which, in the case of the resident, is only reached by slow degrees; and by closely observing the behaviour as it is presented to us in the life of the resident male, we not only gain a better insight into the changes in operation, but can actually witness the breaking down of the winter routine, stereotyped through repetition, by the new disposition as it arises.

The first visible manifestations, even though they may be characterised by a certain amount of vagueness, are therefore of great importance if the behaviour is to be interpreted aright; and in order to insure that none of these earlier symptoms shall be missed, it is necessary to begin the daily record of the bird's movements at an early date in the season. As a rule the second week in February is sufficiently early for the purpose, but the date varies according to the prevailing climatic conditions. Even in species widely remote there is great similarity of procedure, and the behaviour of the Buntings is typical of that of many. With the rise of the appropriate organic state the male resorts at daybreak to a suitable environment, occupies a definite position, and singling out some tree or prominent bush, which will serve as a headquarters, advertises its presence there by song. At first the bird restricts its visits, which though frequent in occurrence are of short duration, for the most part to the early hours of the morning; it disappears as suddenly as it appeared, and one can trace its flight to the feeding grounds—a homestead or perhaps some newly sown field. But by degrees the impulse to seek the society of the flock grows less and less pronounced, the visits to the territory are more and more prolonged, and the occupation of it then becomes the outstanding feature of the bird's existence. This in outline is the course of procedure as it appears to an external observer.

But although much can be learnt from the lives of these smaller species, there is no gain-saying the fact that a great deal of patient observation is required, and the process is apt to become tedious. There are others, however, which are more readily observed, whilst their life-histories afford just as clear an insight into the effect produced by the new disposition upon the developing situation; and among these the Lapwing takes a prominent position, because it is plentiful and inhabits open ground where it is easily kept in view.

Plans of the Water-meadow showing the Territories occupied by Lapwings in 1915.

Plans of the Water-meadow showing the Territories occupied by Lapwings in 1916

There is a water meadow with which I am familiar, where large numbers resort annually for the purpose of procreation. Here they begin to arrive towards the end of February, and at first collect in a small flock at one end of the meadow. A male, here and there, can then be seen to break away from the flock, and to establish itself in a definite position upon the unoccupied portion of the ground, where it remains isolated from its companions. Others do likewise until the greater part of the meadow is divided into territories. Six of these territories I kept under observation for approximately two months in the year 1915. The occupant of the one marked No. 6 upon the 1915 plan was a lame bird, a fortunate occurrence as it enabled me to follow its movements with some accuracy; and though it maintained its position for some weeks, it ultimately disappeared, as a result, I believe, of the persistent attacks of neighbouring males. The behaviour of the males during the first fortnight or so after they broke away from the flock was interesting. Though they retired to their territories and remained in them for the greater part of their time, yet it was only by degrees that they finally severed their connection with the flock, for so long as a nucleus of a flock remained, so long were they liable to desert their territories temporarily and to rejoin their companions.

Lapwings, as is well known, collect in flocks during the winter months, and these flocks, which sometimes reach vast proportions, are to be found on tidal estuaries, water meadows, arable land, and such like places, according to the prevailing climatic conditions. This flocking may contribute towards survival, and may therefore be the result of congenital dispositions which have been determined on biological grounds. On the other hand, since food at that season is only to be obtained in a limited number of situations, the birds may be simply drawn together by accident. In the former case the behaviour would be instinctive, in the latter, though accidental at first, recurrent repetition would tend to make it habitual; but in either case the impulse to accompany the flock must be a powerful one, for on the one hand it would depend upon inherited, and on the other hand upon acquired, connections in the nervous system. Now observe that soon after the flock arrived in the meadow, single males detached themselves; there was no hesitation, they just retired from their companions and settled in their respective territories. They were not expelled, for if their leaving had been compulsory much commotion would have preceded their departure, and their return would certainly not have been welcomed. A reference to the plan will make the position clearer; the neutral zone inhabited by the flock is there shown as situated in one corner of the meadow, the territories that fell under observation are plotted out as far as possible to scale, and the more important zones of conflict are also marked.

The males spent part of their time in their respective territories and part with the flock, so long as it remained in existence. When a male was in its territory it avoided companions and was openly hostile to intruders; when it was with the flock it wandered about with companions in search of food. The contrast between the two modes of behaviour was very marked, and it was evident that the gregarious instinct was gradually yielding its position of importance to the new factor—the territory. If there had been no flock, if a few solitary individuals had appeared here and there and had established themselves in different parts of the meadow, one would have had no definite evidence of the strength of the impulse in the male to seek a position of its own, one could only have argued from the general fact of males flocking in the winter and isolating themselves in spring that something more than accident was required to explain so radical a change. But since the birds returned in a flock to the ground upon which they intended to breed, and since the flock occupied temporarily part of the ground whilst the partitioning of the remainder was still proceeding, it was possible to gauge the strength of the impulse, which was forcing the males to isolate themselves in particular areas of ground, by comparing it with the impulse to accompany the flock—and the measure of its intensity was the rapidity with which the latter impulse yielded its position of importance.

Like the Lapwing, the Coot and Moor-Hen are easily kept under observation, and since many individuals often breed in proximity, more than one can be watched at the same moment; moreover the area occupied by each male generally embraces an open piece of water as well as part of the fringe of reeds, so that the movements of the bird can be followed without much difficulty. Under favourable conditions manifestations of the developing situation become visible at a comparatively early date in the season—the middle or the latter part of February—and these manifestations resemble those of other species. But the Moor-Hen passes summer and winter alike in the same situation, and being therefore in a position to respond at once to internal stimulation, however vague, the change from the one state to the other is gradual. This, however, is a matter of detail; the main consideration lies in the fact that the impulse to retire to a definite position, to avoid companions, and to live in seclusion, is strongly marked, and produces a type of behaviour similar on the whole to that of the Lapwing. First of all there is the appropriation of a certain position, the limits of which are fixed according to the law of habit formation, and according to the pressure exerted by neighbouring individuals; then there is the neutral ground over which the birds wander amicably in search of food; and finally there is the contrast between the pugnacity of the male whilst in its territory, and its comparative friendliness when upon neutral ground.

Evidence of similar behaviour is to be found in the life of the Black Grouse, a bird which has always excited the curiosity of naturalists on account of the special meeting places to which both sexes resort in the spring. Mr. Edmund Selous watched these birds in Scandinavia, where he kept a daily record at one of the meeting places. In various passages he refers to the appropriation of particular positions by particular males, and concludes thus: "It would seem from this that, like the Ruffs, each male Blackcock has its particular domain on the assembly ground, though the size of this is in proportion to the much greater space of the whole. On the other mornings, too, the same birds, as I now make no doubt they are, have flown down into approximately the same areas."

The cliff-breeding species—Guillemots, Razorbills, and Puffins—are difficult to investigate because individuals vary so little, and the sexes resemble one another so closely; yet, despite these difficulties, we can gain some idea of the general purport of their activities. But when the ledges are crowded and the air is filled with countless multitudes, how is it possible to keep a single bird in view for a sufficient length of time to understand its routine? The difficulty is not an insuperable one. The flights, undertaken seemingly for no particular purpose, are often of short duration and are completed before the strain of observation becomes too great; moreover an individual sometimes possesses a special mark or characteristic which serves to make it conspicuous. For example, there is a well-marked variety of the Common Guillemot, the Ringed or Bridled Guillemot of science, distinguished by an unusual development of white round the eye and along the furrow behind it. One such individual I was fortunate in discovering upon a crowded cliff, and, as in the case of the Lapwing with the broken leg or the Yellow Bunting with the injured foot, the identity of the bird was beyond dispute, and one could observe that it appropriated to itself a particular position upon a particular ledge.

Guillemots and Razorbills return at intervals to the breeding stations early in the season, and these visits are repeated with growing frequency until the birds are finally established. I have witnessed these periodic returns during March in the south of England, and during April in the north-west of Ireland, and I am informed that in the latter district such visits may occur as early as February. Gätke, who had ample opportunity of observing the birds in Heligoland, puts their return at an even earlier date. "They visit their breeding places," he says, "in flocks of thousands at the New Year, often even as early as December, as though they wanted to make sure of their former haunts being well preserved and ready for their reception." Such visits, however, are irregular in occurrence; the birds arrive, and, after spending a short time upon the ledges, disappear. And since there is not the same evidence in their coming and going of that method which we observe in the periodical returns of the Bunting or the Finch, it may be thought that needless importance is being attached to an episode in their lives which is quite intelligible in terms of a feeble response determined by a dawning organic change. While it may be quite intelligible in such terms it is not thereby explained; for every response must have as its antecedent an inherited connection in the nervous system determined on biological grounds. Besides, these early periodic returns conform in general to the type of behaviour displayed by other species, the males of which return to their breeding grounds many weeks before the real business of reproduction begins. Are we then justified in regarding them as accidents of the developing situation? Are we not rather bound to admit that they have some definite biological end to serve?

These examples show that the males of many species reverse their mode of life at the commencement of the breeding season and proceed to isolate themselves, each one in a definitely delimited area.

There are three ways in which we may attempt to interpret this particular mode of male behaviour. We may regard it as an accidental circumstance, nowise influencing the course of subsequent procedure; or, appealing to the law of habit formation, we may regard it as an individual acquirement; or again, we may invest it with a deeper significance and seek its origin in some specific congenital disposition determined on purely biological grounds.

Which of these three shall we choose? The first by itself requires but little consideration; for though it might explain the initial visit, it cannot account for the persistency with which the plot of ground is afterwards resorted to. Supposing, however, that we combine the first and the second; supposing, that is to say, we assume, for the purpose of argument, that the initial visit is fortuitous, and that constancy is supplied by habit formation—would that be a satisfactory interpretation? It is a simple one, inasmuch as it only requires that a male shall alight by chance in a particular place for a few mornings in succession in order that the process may be set in motion. Now an essential condition of habit formation is recurrent repetition; given this repetition and, it is true, any mode of activity is liable to become firmly established. But how can we explain the repetition? Even if we are justified in assuming that the initial visit is purely an accidental occurrence, we cannot presume too far upon the laws of chance and assume that the repetition, at first, is also fortuitous.

So that we come back to the congenital basis, the last of our three propositions. And it will, I think, be admitted that the facts give us some grounds for believing that the securing of the territory has its root in the inherited constitution of the bird. In comparing the behaviour of the migratory male with that of the resident, attention was drawn to the manner in which the occupation of a territory was effected: the former bird, it may be remembered, established itself without delay, whereas the latter did so only by degrees, and the difference was attributed to the incidence of migration which required a closer correspondence between organic process and external environment. But the significance for us just now lies in the fact that the definiteness, which accompanies the initial behaviour of the migratory male in relation to the territory, cannot have been acquired by repetition; for this reason, that when the male occupies its space of ground at the end of its long and arduous journey, it does so without preparation or experiment, even without hesitation, as if aware that it was making good the first step in the process of reproduction. No doubt, if it happened to be an individual that had already experienced the enjoyment of reproduction, it might be aware of the immediate results to be achieved and act accordingly. But among the hosts of migrants that one observes, there must be many males which have not previously mated; and yet, upon arrival, they all behave in a similarly definite manner—so that experience cannot well be the primary factor in the situation. If, then, the essential condition of habit formation is absent and experience is eliminated, there is nothing left but racial preparation to fall back upon.

Nevertheless, it is true that many resident males seem to pass through a period of indecision before they establish themselves permanently in their respective territories; they come and go, their visits grow more and more prolonged, and only after the lapse of some considerable time does the process of establishment attain that degree of completeness which is represented in the initial behaviour of the migratory male. Their whole procedure seems therefore to bear the stamp of individual acquirement; and, if it stood alone, we might be content to construe it thus, but the example of the migratory male necessitates our looking elsewhere for the real meaning of the indecision.

Let me first of all give some instances of the persistence with which a male remains in one spot, and this despite the fact that it has no mate.

A Reed-Bunting occupied a central territory in a strip of marshy ground inhabited annually by four or five males of this species. Throughout April, May, and until the 19th June, it clung to its small plot of ground, tolerated no intrusion, and sang incessantly.

Two Whitethroats arrived at much the same time—the 30th April approximately—and occupied the corner of a small plantation; the one obtained a mate the day following its arrival, the other remained unpaired for a fortnight.

A Reed-Warbler established itself amongst some willows and alders adjoining a reed-bed and made its headquarters in a small willow bush. Not more than fifteen yards away, on the edge of the main portion of the reeds, another male was established and was paired on the 22nd May. Each morning the single male behaved in much the same way, singing continuously whilst perched upon the bush. And so the days passed by until it seemed improbable that it would ever secure a mate, but one appeared on the 20th June, and a nest was built forthwith.

Now it is difficult to believe that a chance visit, even though repeated for a few mornings in succession, could have accounted for the Reed-Bunting remaining so persistently in the marsh, or the Whitethroat in one corner of the osier bed, or the Reed-Warbler in that one particular willow. Not only so, but if a habit of such evident strength can be acquired so readily, we have a right to ask why it should only be acquired in the spring—why not at every season? Considerations such as these lead to the belief that there must be some congenital basis to account for such persistent endeavour; the more so since it is difficult not to be impressed with the conative aspect of the male's behaviour. To a stranger, unacquainted with its previous history, the bird might appear to be leading a life of hesitation, whereas, if carefully watched, its whole attitude will be found to betray symptoms of a striving towards some end; and the frequent departure and return, which might be pointed to as the material from which a definite mode of procedure would be likely to emerge, is in reality behaviour of a determinate sort.

My interpretation, then, of the apparent indecision in the behaviour of the resident male is this. During the winter most species live in societies, together they seek their food and together they retire in the evening to the accustomed roosting places; and the association of different individuals confers mutual benefits upon the associates. The movements of these societies are dominated by the question of food; all else is subservient, and the supply of the necessary sustenance may, under certain conditions, become a difficulty which can only be met by energy and resource. After the long night the sensation of hunger is strong, and the birds, on awakening, fly to the accustomed feeding grounds, returning again in the evening to the selected spot, and by frequent repetition a routine becomes established. Thus the behaviour of each individual is determined not only by the powerful gregarious impulse but also by the habits formed in connection therewith during many weeks in succession. Now with the rise of the appropriate organic state, the disposition to seek the breeding ground and there to establish itself becomes dominant in the male. But the process is a gradual one. There is no need, as happens amongst the migrants, for the period of organic change to conform rigidly to the growth of any particular condition in the environment, and hence for a time the bird oscillates between two modes of behaviour—between that one organised by frequent repetition and that one determined by the functioning of this new disposition.

To look at the matter broadly, it is scarcely likely that so definite a mode of behaviour would recur with such regularity, generation after generation, in the individuals belonging to so many widely divergent forms, if it had no root in the inborn constitution of the bird. But the law of habit formation has its part to play also. By itself it is inadequate; yet it probably does assist very materially in adding still greater definition, and it probably is responsible in a large measure for determining the limits of the territory according to the conditions of existence of the species—thus the Falcon seeks its prey over wide tracts of land, and, by hunting over certain ground repeatedly, establishes a routine, which broadly fixes the area occupied; the Woodpecker cannot find food upon every tree, and every forest does not contain the necessary trees, and therefore the bird regulates its flight according to the position of the trees; and the Warbler, finding food close at hand, does not need to travel far, and the area it occupies is consequently small.

So that the most likely solution of the problem will be found in a combination of our second and third propositions; that is to say, in an initial responsive behaviour provided for in the inherited constitution of the nervous system, and in a definiteness acquired by repetition and determined by relationships in the external environment.

CHAPTER III

THE DISPOSITION TO DEFEND THE TERRITORY

In the previous chapter I endeavoured to show that each male establishes a territory at the commencement of the breeding season, and there isolates itself from members of its own sex. And further I gave my reasons for believing that this particular mode of behaviour is determined by the inherited nature of the bird, and that we are justified in speaking of it as "a disposition to secure a territory" because we can perceive its prospective value. But the act of establishment is only one step towards "securing." By itself it can achieve nothing; for any number of different individuals might fix upon the same situation, and if there were nothing in the inherited constitution of the bird to prevent this happening, where would be the security, or how could any benefit accrue to the species?

In withdrawing from its companions in the spring, the male is breaking with the past, and this action marks a definite change in its routine of existence. But the change does not end in attempted isolation; it is carried farther and extends to the innermost life and affects what, humanly speaking, we should term its emotional nature, so that the bird becomes openly hostile towards other males with whom previously it had lived on amicable terms.

The seasonal organic condition is responsible for the functioning of the disposition which results in this intolerance, just as it is for the functioning of the disposition which leads to the establishment of the territory; and the effect of these two dispositions is that a space of ground is not only occupied but made secure from intrusion. The process is a simple one. There is no reason to believe, there is no necessity to believe, that any part of the procedure is conditioned by anticipatory meaning; the behaviour is "instinctive" in Professor Lloyd Morgan's definition of the word, since it is of a "specific congenital type, dependent upon purely biological conditions, nowise guided by conscious experience though affording data for the life of consciousness."

That the males of many animals are apt to become quarrelsome during the mating period is notorious. Darwin collected a number of facts, many of which related to birds, showing the nature and extent of the strife when the sexual instinct dominated the situation. And pondering over these facts, he deduced therefrom a "law of battle," which, he believed, bore a direct relation to the possession of a female. And it must be admitted that he had excellent ground for his conclusion in the fact not only that the conflicts occur mainly during the pairing season, but that the female is often a spectator and seems even to pair with the victor. I accepted it, therefore, as the most reasonable interpretation of the facts. But, as time passed by, incidents of a conflicting character led me to think that after all there might be another solution of the problem. And when it was no longer possible to doubt that there was a widespread tendency to establish territories, it at once became manifest that the battles might have an important part to play in the whole scheme. But how was this to be proved? What sort of evidence could show whether the proximate end for which the males were fighting had reference to the female or to the territory? Clearly nothing but a complete record of the whole series of events leading up to reproduction could supply the necessary data upon which a decision might rest. In the present chapter I shall give, in the first place, the reasons which lead me to think that the origin of the fighting cannot be traced to the female; afterwards, the evidence which seems to show that it must be sought in the territory; and finally, I shall make a suggestion as to the part the female may play in the whole scheme.

The facts upon which the "law of battle" was founded were ample to establish the truth of its main doctrine. But the evidence upon which the interpretation of the battles was based was somewhat superficial. It was based mainly upon the general observation that one or more females could frequently be observed to accompany the combatants; and if this were the sole condition under which the fighting occurred, one must admit that this view would have much to recommend it. But it is not merely a question of males disputing in the presence of a female; for males fight when no female is present, pair attacks pair, or a male may even attack a female—in fact there is a complexity of strife which is bewildering.

In attributing the rivalry to the presence of the female, it is assumed that males are in a preponderance, and that consequently two or more are always ready to compete for a mate. Her presence is presumably the condition under which his pugnacious nature is rendered susceptible to its appropriate stimulus, the stimulus being, of course, supplied by the opponent. There would be nothing against this interpretation if it were in accord with the facts; but it can, I think, be shown that the males are just as pugnacious and the conflicts just as severe even when the question of securing a mate is definitely excluded; and I shall now give the evidence which has led me to this conclusion.

In the previous chapter we had occasion to refer to the difference in the times of arrival of the male and female migrants, and we came to the conclusion, it may be remembered, that this was a fact of some importance, because it gave us a clue to the meaning of much that was otherwise obscure in their behaviour. But it is also of importance in connection with the particular aspect of the problem which we now have in view, for if it can be shown that males, when they first reach their breeding grounds, are even then intolerant of one another's presence, if their actions and attitudes betray similar symptoms of quasi-conation, if disputes are rife and the struggles of a kind to preclude all doubt as to their reality, then it is manifest that in such cases their intolerance cannot be due to the presence of the female.

Here, however, I must refer to a view which is held by some psychologists, namely, that amongst the higher animals, even on the occasion of the first performance of an instinctive act, there is some vague awareness of the proximate end to be attained. Discussing the nature of instincts, Dr M'Dougall[3] says, "Nor does our definition insist, as some do, that the instinctive action is performed without awareness of the end towards which it tends, for this, too, is not essential; it may be, and in the case of the lower animals no doubt often is, so performed, as also by the very young child, but in the case of the higher animals some prevision of the immediate end, however vague, probably accompanies an instinctive action that has often been repeated." A similar view seems to be held by Dr Stout.[4] "As I have already shown," he says, "animals in their instinctive actions do actually behave from the outset as if they were continuously interested in the development of what is for them one and the same situation or course of events; they actually behave as if they were continuously attentive, looking forward beyond the immediately present experience in preparation for what is to come. They apparently watch, wait, search, are on the alert. They also behave exactly as if they appreciated a difference between relative success and failure, trying again when a certain perceptible result is not attained and varying their procedure in so far as it has been unsuccessful. All these characters are found in the first nest-building of birds as well as in the second; they are found also in courses of conduct which occur only once in the lifetime of the animal." Both these writers would, I imagine, contend that, even when a female is absent, the idea of the female, as the end in view throughout, is present; and they would argue that the fact of her absence during the fighting in no way disposes of the belief that she is the condition under which the pugnacious instinct of the male is rendered susceptible to stimulation. What reason is there to think that this interpretation is applicable to the case under consideration? When a female is present, we observe that the males are pugnacious, and, when she is absent, that they still continue to be hostile—that is to say, they behave as if she were present. Now, as far as I can ascertain, the "as if" is the only ground there is for supposing that the female is represented in imaginal form—there is no evidence of the fact, if fact it be. On the contrary, the behaviour of the male affords some fairly conclusive evidence that no such image is the primary factor in exciting the instinctive reaction. For if it be the actual presence of the female, or, in the absence of such, a mental image, that renders the pugnacious nature of the male responsive; provided the usual stimulus were present, the instinct ought surely to respond, not only under one particular circumstance, but under all circumstances. Yet, as we shall presently see, a male is by no means consistently intolerant of other males. It may be sociable at one moment or pugnacious at another, but the pugnacity is always peculiar to a certain occasion—the occupation of a territory. What shall we say then—that a mental image is a situational item only when the territory is occupied? It may be so; it may be that the fact of occupation gives rise to the mental image which, in its turn, renders the fighting instinct explosive, which again renders the possession of the territory secure. That such an interpretation is possible we must all admit. But if it were true, though it would not affect the main consideration, namely, whether the fighting has reference to the possession of a particular female, or to the protection of the territory, it would make further discussion as to which of these is the condition of the fighting unprofitable, for each would have its part to play in the process, the territory remaining, however, the principal factor in the situation.

Now the difference in the times of arrival of the male and female migrant varies in different species from a few days to a fortnight or even more. It is most marked in those that return to their breeding grounds early in the season, and the greater the margin of difference the greater scope is there for observation. In my records for the past twelve years, there are frequent references to these initial male contests in the life of the Willow-Warbler and of the Chiffchaff; and in the district which I have in mind, these two species arrive early in the season, the males preceding the females by a week or even as much as a fortnight. Suppose, then, that two Chiffchaffs establish themselves in adjoining territories; or suppose that a male settles in a territory already occupied; what is the result? Well, scenes of hostility soon become apparent; as the birds approach one another they become more and more restive, their song ceases, they no longer search for food in the usual methodical manner, but instead their movements are hurried and their call-notes are uttered rapidly—all of which betrays a heightened emotional tone. Then the climax is reached, there is a momentary fluttering of tiny wings, a clicking of bills, and for the time being that may be all. But unless one or other of the combatants retires, this scene may be repeated many times in the course of a few hours, and repeated with varying degrees of severity. Yet the fighting, even in the most extreme form, when the birds locked together fall slowly to the ground, is seldom of an impressive kind, and one has to bear in mind the capabilities of the actors, remembering that the most severe struggle might readily be interpreted as a game if it were not for certain symptoms which reveal its inner nature.

The males of many other migrants can frequently be observed to fight when there was every reason to believe that females had still to arrive. The Blackcap is notoriously pugnacious, but not more so than the Marsh-Warbler or the Whinchat. Here in Worcestershire, the Arundo phragmites grows mainly on certain sheets of water which are comparatively few and far between, and the Reed-Warbler is consequently restricted to isolated and more or less confined areas. The males arrive early in May before the new growth of reeds has attained any considerable height, and each one has its own position in the reed-bed, sings there, and throughout the whole period of reproduction actively resists intrusion on the part of other males. I have kept watch upon a small area of reeds daily from the date of the first arrival; each individual was known to me, and as the growing reeds were only a few inches in height, a female could scarcely have escaped detection. Yet time and again disputes arose, and males pursued and pecked one another, striving to attain that isolation for which racial preparation had fitted them.

But on account of their violence, or their novelty, or because the absence of a female was beyond question, some battles stand out in one's memory more prominently than others. An instance of this was a struggle between two Whitethroats which happened in the latter part of April and lasted for three successive days. The scene of its occurrence was more or less the same on each occasion, and the area over which the birds wandered was comparatively small. The fighting was characterised by persistent effort and was of a most determined kind, and so engrossed did the assailants become that they even fluttered to the ground at my feet. No trace of a female was to be seen at any time during these three days, nor, during the pauses in the conflict, was the emotional behaviour of a kind which led me to suppose that a female was anywhere in the vicinity. And, if she had been near, she must have made her presence known, for the belief that she is a timid creature, skulking on such occasions in the undergrowth, is by no means borne out by experience.

Even more impressive was a battle between two male Cuckoos. It occurred high up in the air above the tops of some tall elm-trees which roughly marked the boundary line between their respective areas, and the actions of the birds were plainly visible. At the moment of actual collision the opponents were generally in a vertical position, and wings, feet, and beaks were made use of in turn; one could plainly see them strike at one another with their feet, and one could observe the open bill which generally denotes exhaustion, but may of course have been due to anger, or used as a means of producing terror. Yet no female appeared in the locality until six days after the occurrence of this struggle—and she certainly is not easily overlooked, for her note is unmistakable even when the behaviour of the male does not betray her arrival.

That the actual presence of the respective females exercised any influence on the course of these struggles is more than doubtful. Not only did one fail to detect them, but one's failure to do so was confirmed by the knowledge that they had not yet arrived in those particular localities. Hence the fact of the male preceding the female is a valuable aid to the interpretation of subsequent behaviour; and one appreciates it the more after having experienced the difficulty of deciding whether she is present during the conflicts between resident males, for no matter how carefully we may observe the conditions which lead up to, and which accompany, such conflicts, or how closely we may scrutinise the surrounding trees, undergrowth, or ground, there always remains the possibility that she may, after all, have been overlooked. But this must not be taken to imply that in such cases direct observation alone can lead to no serviceable result, or that the evidence gained therefrom is worthless. Far from it. Failure to detect a female is so very common an occurrence that, even if we lacked the corroborative evidence supplied in the life of the migratory male, it would still be unreasonable to suppose that it were solely due to mistaken observation. We mark her absence during the conflicts between the respective males of many common species—the Finches, Buntings, and Thrushes that occupy their territories early in the season when the hedgerows and trees are still bare; but more frequently amongst those that inhabit open ground, because the movements of the birds are there more accessible to observation. For instance, half a dozen or more Lapwings can be kept in view at the same time, and as they stand at dawn in solitary state, keeping watch upon their respective territories, they are conspicuous objects on the short, frosted grass; no stranger can enter the arena without the observer being aware of it, no commotion can occur but one detects it, no movement however small need be missed. And so they fight, in a manner which leaves no doubt as to the reality of the struggle, when their prospective mates are absent not only from the particular territories in which the conflicts take place, but absent too from those adjoining.

If the fact that males fight before they are paired and in the absence of a female could be placed beyond all question, it would no longer be possible to regard her possession as the end for which they are contending, and consequently there would be no need to produce further evidence. But the examples which I have given refer, of course, to only a few migrants and a few residents—and moreover it must be admitted that a female is often conspicuous during the battles—so that by themselves they must be regarded, and rightly so, as inconclusive. We must therefore pass on to consider evidence of a somewhat different character.

I spoke of the complexity of the strife. By this I mean that it is not merely a matter of disputes between adjoining males, but that it is a far more comprehensive business involving both sexes. Thus female fights with female and pair with pair, or a male will attack a female, or, again, a pair will combine against a single male or a single female. And from all this complexity of strife we gain much valuable evidence in regard to the question immediately before us. For when one pair attacks another, or males that are definitely paired fight with one another, or an unpaired male attacks either sex of a neighbouring pair indiscriminately, there is surely little ground for supposing that the possession of a mate is the reason of it all.

The battles between pairs of the same species are by no means uncommon. Observe, for example, the central pair of three pairs of Reed-Buntings occupying adjoining territories, and keep a daily record of the routine of activity practised by both sexes during the early hours of the morning; then, at the close of the season, summarise all the fighting under different headings, and it will be found that the number of occasions upon which the central pair attacked, or was attacked by, neighbouring pairs will form a considerable portion of the whole.

Or watch the Moor-Hen, and for the purpose choose some sheet of water large enough to accommodate three or more pairs, and so situated that the birds can always be kept in view. Early in February the pool will be haunted by numbers of individuals of both sexes, all swimming about together, and, if the pool is surrounded by arable land, wandering over that land subject to no territorial restrictions, apparently free to seek food where they will. But as time goes by, their number gradually decreases until a few pairs only remain, and these will occupy definite areas. If careful watch is then kept and the relations of the pairs closely studied, there will be no difficulty in observing the particular kind of warfare to which I am alluding, and it will be noticed that the encounters are of a particularly violent description. Thus two pairs approach one another, and, when they meet, throw themselves upon their backs, each bird striking at its adversary with its feet or seizing hold of it with its beak; and though, in the commotion that ensues, it is almost impossible to determine what exactly is happening, there is reason to believe that the sexes attack one another indiscriminately.

A struggle between two pairs of Pied Wagtails is worth mentioning. It impressed itself upon my memory because of the unusual vigour with which it was conducted. The battle lasted for fifteen minutes or more, and the four birds, collecting together, pursued and attacked one another—at one moment in the air, at another upon the roof of a house where they would alight and flutter about on the slates, uttering their call-note without ceasing—until finally they disappeared from view, still, however, continuing the struggle.

Such is the nature of the warfare which prevails between neighbouring pairs, and which can be observed in the life of many other species—the Chaffinch, Stonechat, Blackbird, Partridge, Jay, to mention but a few.

The conflicts between males that are definitely paired are of such common occurrence that it is scarcely necessary to mention specific instances. But the occasions on which a male attacks either sex of a neighbouring pair indiscriminately, or on which a pair combine to attack a female, are less frequent.

Now if it be true that males fight for no other purpose than to gain possession of a mate, what meaning are we to attach to the battles between the pairs, or what explanation are we to give of the fact that paired males are so frequently hostile? Those who hold this view will probably argue thus: "The presence of the female is the condition under which the pugnacious instinct of the male is rendered susceptible to appropriate stimulation, and the stimulus is supplied by a rival male; we admit that all the fighting which occurs after pairing has taken place has nothing to do strictly speaking with gaining a mate, but, inasmuch as the fact of possession is always liable to be challenged—and no male can differentiate between a paired and an unpaired intruder—we contend that it would add to the security of possession if the pugnacious instinct remained susceptible to stimulation so long as there were any possibility of challenge from an unpaired male; and we think that the waste of energy involved in the struggles between paired birds, and which we grant is purposeless, would be more than balanced by the added security." This is a possible explanation and requires consideration. It cannot account for all the diverse ways in which the sexes are mixed up in the fighting—it cannot, for instance, explain the fact that an unpaired male will attack either sex of an adjoining pair indiscriminately—but nevertheless it appears at first sight to be a reasonable explanation of some of them. We must remember, however, that fighting continues throughout the whole period of reproduction. Even after the discharge of the sexual function has ceased, and the female is engaged in incubation or in tending her young, the male is still intolerant of intruders; and it is difficult to believe that, at so late a stage in the process, a female could be any attraction sexually to an unpaired male. But apart from any theoretical objection, there remains the fact—namely that there is no evidence that a male, after having once paired, is liable to be robbed of its mate. And in support of this fact I have only to state that I have met with no single instance of failure to obtain and hold a mate when once a territory had been secured. Bearing in mind then that both sexes participate in the fighting, and that individuals of the opposite sex frequently attack one another; that all such conflicts are characterised by persistent effort, and that they are not limited to just the particular period when the sexual instinct is dominant but continue throughout the breeding season; bearing in mind that in at least one form of this promiscuous warfare the influence of the female can be definitely excluded, and that, in the remaining forms, the evidence which is required to link them up with the biological end of securing mates is lacking—can it be denied that the complexity of the strife makes against the view that the possession of a female is the proximate end for which the males are fighting?

We started with the most simple aspect of the whole problem, the fighting of two males in the presence of one female—the aspect upon which attention has usually been fixed. And if it remained at that, if observation failed to disclose any further development in the situation, then there would be no need to probe the matter deeper, there would be no reason to doubt the assertion that the quarrel had direct reference to the female. But assuredly no one can ponder over the diversity of battle and still believe that the possession of a mate furnishes an adequate solution of the mystery. Clearly such an hypothesis cannot cover all the known facts; there are conflicts between separate pairs, and there are conflicts between males when females are known to be absent and when their mates are even engaged in the work of incubation—these cannot be due to an impulse in a member of one sex to gain or keep possession of one of the other sex. So that taking all these facts into consideration, we are justified, I think, in hesitating to accept this view, and must look elsewhere for the real condition under which the pugnacious nature of the male is rendered susceptible to appropriate stimulation.

What then is the meaning of all this warfare? The process of reproduction is a complex one, built up of a number of different parts forming one inter-related whole; it is not merely a question of "battle," or of "territory," or of "song," or of "emotional manifestation," but of all these together. The fighting is thus one link in a chain of events whose end is the attainment of reproduction; it is a relationship in an inter-related process, and to speak of it as being even directly related to the territory is scarcely sufficient, for it is intimately associated with the disposition which is manifested in the isolation of the male from its companions, and forms therewith an imperium in imperio from which our concept of breeding territory is taken. But let me say at once that it is no easy matter to prove this, for since so many modes of behaviour, which can be interpreted as lending support to this view, are likewise interpretable on the view that the presence of a female is a necessary condition of the fighting, it is difficult to find just the sort of evidence that is required. Nevertheless, after hearing the whole of the evidence and at the same time keeping in mind the conclusion which we have already reached, I venture to think that the close relationship between the warfare on the one hand and the territory on the other will be fully admitted.

Formerly I deemed the spring rivalry to be the result of accidental encounters, and I believed that an issue to a struggle was only reached when one of the combatants succumbed or disappeared from the locality, a view which neither recognised method nor admitted control. Recent experience has shown, however, that I was wrong, and that there is a very definite control over and above that which is supplied by the physical capabilities of the birds.

Let us take some common species, the Willow-Warbler being our first example; and, having found three adjoining territories occupied by unpaired males, let us study the conflicts at each stage in the sexual life of the three individuals, observing them before females have arrived upon the scene, again when one or two of the three males have secured mates, and yet again when all three have paired. Now we shall find that the conditions which lead up to and which terminate the conflicts are remarkably alike at each of these periods. A male intrudes, and the intrusion evokes an immediate display of irritation on the part of the owner of the territory, who, rapidly uttering its song and jerking its wings, begins hostilities. Flying towards the intruder, it attacks viciously, and there follows much fluttering of wings and snapping or clicking of bills. At one moment the birds are in the tree-tops, at another in the air, and sometimes even on the ground, and fighting thus they gradually approach and pass beyond the limits of the territory. Whereupon a change comes over the scene; the male whose territory was intruded upon and who all along had displayed such animosity, betrays no further interest in the conflict—it ceases to attack, searches around for food, or sings, and slowly makes its way back towards the centre of the territory.

Scenes of this kind are of almost daily occurrence wherever a species is so common, or the environment to which it is adapted so limited in extent, that males are obliged to occupy adjacent ground. The Moor-Hen abounds on all suitable sheets of water, and it is a bird that can be conveniently studied because, as a rule, there is nothing, except the rushes that fringe the pool, to hinder us from obtaining a panoramic view of the whole proceedings, and moreover the area occupied by each individual is comparatively small. Towards the middle of February, symptoms of sexual organic change make themselves apparent, and the pool is then no longer the resort of a peaceable community; quarrels become frequent, and as different portions of the surface of the water are gradually appropriated, so the fighting becomes more incessant and more severe. Each individual has its own particular territory, embracing a piece of open water as well as a part of the rush-covered fringe, within which it moves and lives. But in the early part of the season, when the territories are still in process of being established, and definiteness has still to be acquired, trespassing is of frequent occurrence, and the conflicts are often conspicuous for their severity.

Now these conflicts are not confined to unpaired individuals, nor to one sex, nor to one member of a pair—every individual that has settled upon the pool for the purpose of breeding will at one time or another be involved in a struggle with its neighbour. If then we single out certain pairs and day by day observe their actions and their attitude towards intruders, we shall notice that, instead of their routine of existence consisting, as a casual acquaintance with the pool and its inmates might lead us to believe, of an endless series of meaningless disputes, the behaviour of each individual is directed towards a similar goal—the increasing of the security of its possession; and further, if we pay particular attention to the circumstances which lead up to the quarrels and the circumstances under which such quarrels come to an end, we shall find, when we have accumulated a sufficient body of observations, that the disputes always originate in trespass, and that hostilities always cease when the trespasser returns again to its own territory. By careful observation it is possible to make oneself acquainted with the boundaries—I know not what other term to use—which separate this territory from that; and it is the conduct of the birds on or near these boundaries to which attention must be drawn. A bird may be feeding quietly in one corner of its territory when an intruder enters. Becoming aware of what is happening it ceases to search for food, and approaching the intruder, at first swimming slowly but gradually increasing its pace, it finally rises and attacks with wings and beak, and drives its rival back again beyond the boundary. Thereupon its attitude undergoes a remarkable change; ceasing to attack, but remaining standing for a few moments as if still keeping guard, it betrays no further interest in the bird with which a few seconds previously it was fighting furiously. On one occasion I watched a trespasser settle upon a conspicuous clump of rushes situated near the boundary. The owner, who was at the moment some distance away, approached in the usual manner, and, having driven off the trespasser, returned immediately to the clump, where it remained erect and motionless.

A feature which marks all the fighting, and which we cannot afford to disregard, is the conative aspect of the behaviour of the owner of the territory. The bird attacks with apparent deliberation as if it were striving to attain some definite end. I recollect an incident which was interesting from this point of view. A pair of Reed-Buntings were disturbed by a Weasel which had approached their nest containing young. Both birds betrayed symptoms of excitement; as the Weasel threaded its way amongst the rushes, so they fluttered from clump to clump or clung to the stems, uttering a note which is peculiar to times of distress, and followed it thus until finally it disappeared in a hedge. The rapidly uttered note and the excitement of the birds caused some commotion, and the male from an adjoining territory approached the scene. Now one would have expected that the presence of this bird, and possibly its aid in driving away a common enemy, would have been welcomed; one would have thought that all else would have been subservient to the common danger, and that so real a menace to the offspring would have evoked an impulse in the parent powerful enough to dominate the situation and subordinate all the activities of the bird to the attainment of its end. But what happened? Three times during this incident, the male, whose young were in danger, abandoned the pursuit of the Weasel and pursued the intruder. It was not merely that he objected to the presence of this neighbouring male in a passive way, nor even that he had a momentary skirmish with it, but that he determinedly drove the intruder beyond the boundary and only then returned to harass the Weasel.

Thus it seems clear that the proximate end to which the fighting is directed is not necessarily the defeat of the intruder, but its removal from a certain position. And inasmuch as this result will be obtained whether the retreat is brought about by fear of an opponent or by physical exhaustion, it is manifest that too much significance need not be attached to the amount of injury inflicted. It is necessary to bear this in mind, because it is held by some, who have carefully observed the actions of various species, that overmuch importance is attached to the conflicts, that in a large number of instances they are mere "bickerings" and lead to nothing, and that they are now only "formal," which means, I suppose, that they are vestigial—fragments of warfare that determined the survival of the species in bygone ages. But if the conclusion at which we have just arrived be correct, if we can recognise a single aim passing through the whole of the warfare—and that one the removal of an intruder from a certain position, then we need no longer concern ourselves as to the degree of severity of the battles—we see it all in true perspective. Neither exhaustion nor physical inability are the sole factors which determine the nature and extent of the fighting; there is a more important factor still—position. According, that is to say, to the position which a bird occupies whilst fighting is in progress, so its pugnacious nature gains or loses susceptibility, and it is this gain or loss of susceptibility which I refer to when I speak of the fighting as being controlled.

What we have then to consider is the relation of "susceptibility" to "position." We can explain the relationship in two ways. We can say that the part of the nature of the male which leads to the occupation of a territory, and is partly hereditary and partly acquired, is stronger than the part which leads the bird to fight, and which is conditioned by the presence of a female, and that consequently when the male passes the boundary, the impulse to return asserts itself and the conflict ceases; or we can say that the occupation of a territory is the condition under which the pugnacious instinct is rendered susceptible to stimulation, that the stimulus is supplied by the intruder, and that when the male passes outside the accustomed area its instinct is no longer so susceptible and it therefore retires from the conflict.

Of these explanations, the first is not altogether satisfactory. It requires the presence of a female and, as we have seen, a female is by no means always present. Then it attributes to the one side of the inherited nature an influence which is not borne out by the facts, for in the ordinary routine of existence, without the incentive of battle, every individual is liable to wander occasionally beyond its boundary and to intrude temporarily upon its neighbours; and this it could scarcely do, providing its nature to remain within the territory were powerful enough to dominate its movements and curtail its activities even during the excitement of an encounter. But there is nothing inherently improbable in the alternative hypothesis, nor anything that is at all inconsistent with the behaviour as observed; on the contrary, if it is admitted, the facts become connected together and exhibit a meaning which they otherwise would not have possessed.

So much for the controlling influence of "position," which alone seems to me sufficient ground for believing that the fighting has reference to the territory. But it is not the whole of the evidence.

Now if it were possible to demonstrate by actual observation that those males which had not established territories were not pugnacious, we should have something in the nature of proof of the correctness of this view. Demonstrative evidence of this kind is, however, unattainable. Yet we can come very near to obtaining it by reason of a peculiar feature which marks the process of acquiring territory—the neutral ground. The Lapwing will serve as an illustration. In the previous chapter I referred to the small flocks that appeared in the accustomed water meadow early in February, and I described how they settled day after day in that meadow, but only in a limited part of it, where they passed their time in rest, in preening their feathers, or in running this way and that lazily searching for food; and how, at length, the flock dwindled by reason of individuals breaking away in order to secure positions on the remaining part of the meadow. Here the neutral ground is adjacent to the territories, and, while still occupied by the flock, is resorted to by the males that had deserted that flock in order to establish those territories.

Suppose now that we have the whole meadow in view from some point of vantage. In front of us are the territories, in the distance the neutral ground; and in each territory there is a solitary male, while on the neutral ground a number of individuals of both sexes are assembled, and move about freely one amongst another. So that the scene presented to view is somewhat as follows: a flat meadow, at one end of which, and at fairly regular intervals, a few solitary individuals are dotted about, each one keeping at a distance from its neighbours; while at the other end a number of individuals are collected together in a comparatively small space, apparently deriving some satisfaction from their close association. That surely is a very remarkable contrast. But let us continue our investigation, first fixing our attention upon the solitary individuals; one is standing preening its feathers, another is squatting upon the ground, a third runs a few yards in this direction then a few yards in that, stimulated apparently by the sight of food, and so on. Moreover, each one keeps strictly to a well-defined area and makes no attempt to associate with its fellows. One of the males, however, whilst roaming backwards and forwards approaches the limit of its territory, and this brings the neighbouring bird, whose boundary is threatened, rapidly to the spot. In an upright position both stand face to face, and the battle then begins; with their wings they attempt to beat one another about the body, with their beaks they aim blows at the head, and in the mêlée wings and legs seem to be inextricably mixed; whilst at intervals, driven backwards by the force of the collision, they are compelled to separate, only, however, to return to the charge—and the sound of beating wings and the feathers that float in the air are tokens of earnestness. Such scenes are of frequent occurrence; but the conflicts vary in intensity, and the circumstances under which they occur vary too, and females come and go without leaving any clue as to their ultimate intentions.

Turning now to the flock one is impressed with the friendship that seems to exist between the various members. There are, it is true, occasional displays of pugnacity which never seem to develop into anything very serious; for instance, one bird will fly at another, and a momentary scuffle is followed by a short pursuit but nothing more—nothing, that is to say, in the least comparable with the battle previously described. Of what is the flock composed? Of members of both sexes. There is no difficulty in assuring oneself that this is so. But is it entirely composed of individuals in whom development has not reached a stage adequate for the functioning of the primary dispositions? No, not entirely; for it will be observed that its number is a fluctuating one, that birds come and go, and, if a close watch is kept upon the different individuals as they leave, it will be noticed that some at least are inmates of the territories at the opposite end of the meadow—the solitary members whose behaviour we were recently watching. This fact is an important one. We were impressed, it may be remembered, with the contrast between the general behaviour of the birds at the opposite ends of the meadow. But now it appears as if the contrast were not between this individual and that, but between the behaviour of the same one under different circumstances. The male, that is to say, which, while in its territory, tolerates the approach of no other male, flies to the flock and is there welcomed by the very individuals with whom a short time previously it had been engaged in serious conflict.

But if the conditions are reversed and the flock happens to settle in an occupied territory, the attitude of the owner towards the flock is very different. In the year 1916 an incident of this kind occurred in the meadow to which reference has already been made. The weather had been exceptionally severe—very cold easterly and north-easterly winds, frost, and frequent falls of snow had affected the behaviour of the Lapwings, and seemed to have checked the normal development of their sexual routine. The males would attempt to establish themselves, and then, when the temperature fell and the ground was covered with snow, would collect again in flocks and follow their winter routine. It was on the 9th March, during one of the spells of milder weather, that the flock on the neutral ground was disturbed and settled mainly in the territory marked No. 3 on the 1916 plan, but partly on that marked No. 2. The owners thereupon began to attack the different members of the invading flock. Fixing attention upon a particular bird whilst ignoring the remainder, the No. 3 male drove it away, and then after a pause drove another away, and so on until by degrees all the invaders were banished, and the No. 2 male did likewise. The interest of this incident lies, however, in the behaviour of the different individuals of which the flock was composed; when attacked they made no real show of resistance, but accepted the situation and left. The will to fight was clearly lacking, yet their presence was a source of annoyance to the owners of the territories. A short time previously a female had accompanied one of the males and was at that time somewhere in the vicinity, but beyond this there was no evidence to show that either of them were paired, and even if the presence of the female were the reason of the pugnacity of the one, it could not well account for that of the other.

The neutral ground does not always happen to be so close at hand as in the case of the meadow referred to. Sometimes the birds will resort to a particular field, attracted probably by a plentiful supply of food, and here they collect and behave as they do during the winter, running this way and that as the fancy takes them, meeting together by accident at one moment, parting at another, according to the direction in which they happen to wander. Of animosity there is little sign; the season might be the middle of winter instead of the middle of March for all the indication there is of sexual development, and yet one knows that they will behave differently when they leave this ground, as presently they will, and return to their territories in the surrounding neighbourhood, and that there each one will fight if necessary to preserve its acre from intrusion.

It would seem, then, from this that the fighting must bear some relation to the particular area of ground in which it occurs; and unless it can be shown that there is some other factor in the external environment of the male, that is the direction in which we must look for the condition under which the instinct is rendered susceptible. One's thoughts turn, of course, to the female, but she too passes backwards and forwards between the territories and the neutral ground, and if her presence were really a conditio sine qua non of the strife, one would like to know why, when she leaves those territories and joins the flock and the males do likewise, similar conflicts should not prevail there also.

Other species have their neutral ground, but the environment seldom affords such facilities for observation as does that of the Lapwing. Even though the Moor-Hens, who are so conspicuously intolerant upon the pool, do feed together amicably upon the meadows adjoining; and the Chaffinch that is so pugnacious in the morning, does seek out the flock later in the day; yet their conditions of existence prevent our obtaining a panoramic view of the whole proceeding, and we have to study each scene separately before discovering that the relationship between intolerance and the territory on the one hand, and friendship and the neutral ground on the other, is just as strong a feature as it is in the behaviour of the Lapwing.

I shall now give a brief account of the conduct of a male Reed-Bunting which by persistent effort established itself late in the season, and I shall do so because its behaviour tends to confirm much that has been said in the preceding pages.

Early in March three male Reed-Buntings occupied a small water meadow overgrown with the common rush, and by the third week all of them were paired. On the 30th March two of the males were unusually pugnacious, and on the following day fighting continued and at times was very severe. Now I knew that the occupants of the ground in which the fighting was taking place were paired, and not doubting that the combatants were the owners of two territories marked for convenience sake Nos. 1 and 2, I was at a loss to understand the meaning of so determined and persistent a struggle. My attention, however, was presently drawn to a third bird, which also joined in the conflict and made the whole situation still more perplexing. This bird, as it soon became clear, was none other than the owner of No. 2 territory, and the one that I had previously regarded as such was a new arrival. On the following day, the 1st April, fighting continued, and in my record for that day there is a note to the effect that "No. 2 female seems to be of no interest to No. 5 male (the new arrival); its purpose seems to be to drive away intruders." On the 2nd April and subsequent days, this bird attacked every other male that approached, and not only maintained its position but ultimately succeeded in securing a mate. Here then we have two territories occupied by two males, both of which had obtained a mate. The relation of these two birds was normal, a month's routine had defined their boundaries, and conflicts were less frequent than formerly. But upon this comparatively peaceful scene a strange male intrudes. Observe the manner of the intrusion. The stranger does not wander about first in this direction and then in that, but acts as if it had some definite end in view, and establishing itself in a small alder bush which it uses as a base or headquarters, it gradually extends its dominion, gains the mastery over the surrounding ground, part of which belonged to No. 1 male and part to No. 2, and finally drives a wedge, so to speak, between the two territories.

How is its behaviour to be explained, and why did its presence cause such commotion? No one could have watched the gradual unfolding of this incident day by day and not have been impressed by the persistent endeavour with which this male maintained its position in one small part of the meadow. This is the first and most important consideration. Then there is the attitude, also significant, which it adopted towards the females; for I take it that, apart from the question of territory, the explanation of its intrusion must be sought in the necessity for securing a mate—that it was attracted by the presence of the females, and that the proximate end of its behaviour was the possession of one of them. But if there is one thing that emerges from the facts more clearly than another it is that the course of its behaviour was in no way influenced by the presence or absence of either of the females. My reasons for saying so are the following: in the first place, it made no attempt to pursue or to thrust its attention upon either one or the other of them; secondly, it even went so far as to attack and drive them away when they approached too closely; and in the third place, when an unpaired female did at length appear, it adopted a different attitude and forthwith paired. And bearing in mind that these two females had already been with their respective mates for some considerable time, and that there was reason to believe that coition had actually taken place, is it likely that any counter-attraction would have proved successful in tempting either of them away from its mate, or probable, if they were the sole attraction, that the intruding male would have been so persistent in remaining? How very much simpler it is to fit the pieces together, if for the time being we ignore the female and fix our attention upon the territory. Each item of behaviour then falls into its proper place, and the fighting which seemed so perplexing and meaningless becomes a factor of prime importance. First of all the male arrives; then it establishes itself in a small alder bush and advertises its presence by song; next, by persistent effort in attacking the neighbouring males, it frees a piece of ground from their dominion; and finally, in proper sequence, a female arrives, pairing takes place, and reproduction is secured.

How then does the whole matter stand? If it were males only that engaged in serious conflict, and if they fought only in the presence of a female, the problem would resolve itself into one simply of obtaining mates. But the warfare extends in a variety of directions, it is not confined to one sex, nor to unpaired individuals, nor need the opponents necessarily be of the same sex; it involves both sexes alike singly or combined. Now the view that the biological end of battle is, in its primary aspect, related to the female, cannot, as we have seen, apply to the conflicts between different pairs, and only by much stretching of the imagination can it be held responsible for the hostility that males frequently display towards females or vice versa. It is valid only for a certain form of warfare. But that form represents, you will say, a large proportion of the whole, which is true; and so long as we ignore the remainder, we might rest content in the belief that we had solved the major part of the problem. But can we ignore the remainder? Can we say that the conflicts between paired males, for example, are simply offshoots of the pugnacious disposition, and have no part to play in the process of reproduction? They recur with marked persistency season after season and generation after generation; they are to be found in species widely remote; they are frequent in occurrence; and no one who had observed them and noted the vigour with which they are conducted, could, I think, conclude that they were meaningless—and be satisfied. They must somehow be explained. So that if anyone thinks fit to maintain that possession of a mate is an adequate explanation of part of the hostilities, it is clearly impossible to regard all the fighting as a manifestation of one principle directed towards a common biological end.

But wherever we extend our researches, we find that the facts give precision to the view that the occupation of a territory is the condition under which the pugnacious instinct is rendered susceptible to stimulation. The Lapwing, when in its territory, displays hostility towards other males of its own species, but when upon neutral ground, treats them with indifference; the Chiffchaff pursues its rival up to the boundary and is then apparently satisfied that its object has been achieved; the cock Chaffinch in March permits no other male to intrude upon its acre or so of ground during the early hours of the morning, but for the rest of the day it joins the flock and is sociable; the Herring-Gull resents the approach of strangers so long as it occupies its few square feet of cliff, but welcomes companions whilst it is following the plough—all of which points to a relation between the territory and the fighting. And this view has at least one merit—it accounts for all the fighting no matter what degree of severity may be reached or in what way the sexes may be involved. The complexity of the strife presents no obstacle; for if the biological end of the fighting is to render the territory, which has already been established, secure from intrusion, each sex will have its allotted part to play at the allotted time: thus the battles between the males before females appear on the scene will decide the initial question of ownership; those between the females will give an advantage to the more virile members and insure an even distribution of mates for the successful males; the constant struggles between paired males will roughly maintain the boundaries and prevent such encroachment as might hamper the supply of food for the young; and the co-operation of male and female in defence of the territory will be an additional safeguard. Each form of battle will contribute some share towards the main biological function of reproduction.

Hitherto we have dealt principally with the male. We have referred, it is true, to the fact that the female co-operates with her mate in order to drive away intruders, but beyond this, we have made no attempt to trace what part, if any, she plays in the whole scheme. We must do so now.

The various steps by which the territory is not only established but made secure from invasion, imply an inherited nature nicely balanced in many directions—first of all the male must be so attuned as to be ready to search for a territory at the right moment; then it must be capable of selecting a suitable environment; and, having established itself, it must be prepared to defend its area from a rival, and to resist encroachment by its neighbours—and if it failed in any one of these respects, it would run the risk of failure in the attainment of reproduction. Each individual has therefore to pass, so to speak, through a number of sieves—the meshes of which are none too wide—before it can have a reasonable prospect of success. This being so, we ask, in the first place, whether the female, too, may not have an eliminating test to pass; and in the second place, whether she may not also assist in furthering the biological end of securing the territory.

Now the answer to the first of these questions will be found to be in the affirmative. Just as, in the securing of a territory, the ultimate appeal is to the physical strength of the male, so, in the course of her search for a mate, the female may be called upon to challenge, or may be challenged by a rival, and the issue is decided by force. My attention was first drawn to this fact by a struggle between two female Whitethroats, which I have described elsewhere. The scene of its occurrence was the corner of a small osier bed occupied by one male, and the females that took part in it had only recently arrived, but the male, an unpaired bird, had been in possession of its territory for some days. The sequel to this struggle, which was protracted and severe, was the disappearance of both females, the male being left without a mate for a further ten days.

Numerous instances have since come under my notice. Hen Chaffinches become so absorbed that they fall to the ground and there continue the struggle. Seizing hold of one another by the feathers of the head, they roll from side to side, and then, without relaxing their grip, lie exhausted—the quickened heart-beat, altered respiration, tightly compressed feathers and partially expanded wings betraying the intensity of the conflict.

As the breeding season approaches, hen Blackbirds grow more pugnacious. Individuals that early in the year have frequented the same spot daily and have even shown every sign of friendship, become openly hostile. For two years in succession I had an opportunity of observing females under such conditions, and of studying the gradual change in their relationship. Each morning at break of day and for some hours afterwards they could be seen in the same place, one following the other as they searched for food first in this direction and then in that, as if they derived some special pleasure from the fact of their companionship. Then a change began to manifest itself. Indications of animosity became apparent; one would run towards the other in a threatening attitude and, in a half-hearted manner, peck at it; and gradually the hostility grew, until the tentative pecking developed into a scuffle and the scuffle into a conflict.

Much fighting also occurs between the females of the Reed-Bunting, and likewise between those of the Moor-Hen, and because these two species are not only common but inhabit respectively open stretches of marshy ground or large sheets of water, the fighting can be readily observed.

Why do the females fight before they are definitely paired? To obtain mates? This certainly seems to be the obvious explanation because any question of securing territory can be excluded; yet if it be true that their sex is numerically inferior, it is difficult to understand the necessity for such strenuous competition. But what is the condition under which the pugnacious instinct of the female is rendered susceptible to stimulation? It cannot be merely the presence of a male ready to breed, for then there would be endless commotion amongst the flocks of Chaffinches or of Lapwings which in March are composed of both sexes, including even males that have secured territories. There must be some other circumstance; and, judging by experience, it is to be found in the territory—a male, that is to say, in occupation of one, is the condition under which the inherited nature of the female is allowed free play. We must bear in mind, however, that the competition between the males is very severe, that large numbers probably fail to pass even this preliminary test, and that only a proportion are in a position to offer to the female the condition under which her process can successfully run its course; so that the presumption is—though it is incapable of demonstration—that there is a competition for such males each recurring season, and that, on the average, the weaker females fail to procreate their kind.

But apart from any direct assistance she may give in driving away intruders, does she in any way help to further the biological end of reproduction? This is a difficult question to answer, and the suggestion I have to make can only apply in those cases in which the territory is occupied throughout the breeding season. Much of the fighting between the males occurs in her presence, and it must be admitted—though it is difficult to speak with any degree of certainty—that such fighting, taken as a whole, bears the stamp of exceptional determination. Let us then grant that the excitement of a male does, under these circumstances, reach a higher level of intensity, and let us see how this will add to the security of the territory. The fact that the male has established itself and obtained a mate is not alone sufficient to accomplish the end for which the territory has been evolved. During the period between the initial discharge of the sexual function and the time when incubation draws to a close, much may happen to prejudice the future of the offspring; there is always the possibility of invasion by an individual whose development is backward or which has been unsuccessful in making good the first step, and, as we saw in the case of the Reed-Bunting, a portion of the ground won may be lost; there is always the danger of gradual encroachment by neighbouring owners; and there is even a possibility that a pair may be so persistently harassed by more virile neighbours as to forsake the locality permanently. If then a male is to attain a full measure of success it must be capable of keeping its boundaries intact up to the time when the young are able to fend for themselves, and consequently it is important that its intolerant nature should remain susceptible to stimulation throughout the greater part of the season.

Does the presence of a female serve to promote this end? Now we know very little of the influence exerted by one sex upon the other. Professor Lloyd Morgan has suggested that the male raises the emotional tone of the female, a suggestion which seems to me in accordance with the facts. There is reason to believe, however, that the converse is also true—namely that the excitement of the male reaches a higher level of intensity when a female is present. Granting then that his emotional tone is raised, how will this affect the question? So great is the difference of opinion as to the part that the emotions play in furthering the life of the individual that one hesitates to accept any particular one. But it seems to be generally admitted that emotion adds to the efficacy of behaviour, and this is the view of Professor Lloyd Morgan. "Whatever may be the exact psychological nature of the emotions, it may be regarded," he says, "as certain that they introduce into the conscious situation elements which contribute not a little to the energy of behaviour. They are important conditions to vigorous and sustained conation." Therefore, if it be true that the female raises the emotional tone of the male, the result will be an increased flow of energy into all the specific modes of behaviour connected with reproduction, amongst which those directly concerned in the securing and defence of the territory will receive their share; so that instead of a progressive weakening of just those elements in the situation which make for success, the level of their efficiency will be maintained as a result of such reinforcement. But the female becomes intolerant of her own sex when she has discovered a male ready to breed, and, later, assists her mate in resisting intrusion; and by raising her emotional tone, he may be the means of furthering more strenuous behaviour on her part. Each member of the pair would in this way contribute towards the energy of behaviour of its mate, and hence add indirectly to the security of the territory.

It may be well to illustrate the foregoing remarks. Suppose that there is a small piece of woodland barely sufficient to hold three pairs of Willow-Warblers, and suppose that the male and female in the middle territory did not respond to one another's influence quite as readily as the adjoining males and females, what would be the result? The emotional tone of the central pair would stand at a lower level of intensity; and, since their congenital dispositions would lack the necessary reinforcement, the birds would tend to become less and less punctilious in keeping their boundaries intact, whereas the adjoining pairs, always on the alert and meeting with little opposition, would encroach more and more and gradually extend their dominion. And so, by the time the young were hatched, the parents would be in occupation of an area too limited in extent to insure the necessarily rapid supply of food, and would be compelled to intrude upon the adjoining ground. But knowing how routine becomes ingrained in the life of the individual, knowing that for weeks this pair had submitted to their neighbours, can we believe that they would be capable of asserting their authority and that the young would be properly cared for? Or suppose that different pairs of Kittiwake Gulls on the crowded ledges, or different pairs of Puffins in the crowded burrows, varied in like manner, would they all have equal chances of rearing their offspring? The struggle for reproduction is nowhere more severe than amongst the cliff-breeding sea birds; it is not for nothing that one sees Kittiwake Gulls, locked together, fall into the water hundreds of feet below and struggle to the point of exhaustion, or, as has been reported, to the point of death; it is not for nothing that Puffins fight with such desperation. And surely success will be attained by that pair whose emotional tone stands high and whose impulse to fight is therefore strong, rather than to the ill-assorted couple.

The argument, then, is briefly this. In the spring, a marked change takes place in the character of the males of very many species; instead of being gregarious they either avoid one another and become hostile, or, if their conditions of existence require that they shall still live together, they become irritable and pugnacious. This change is made known to us by the battles of varying degrees of severity which are such a feature of bird life in the spring; and since a female can commonly be observed to accompany the combatants, the possession of a mate appears at first sight to be the proximate end for which the males are contending. But when the circumstances which lead up to the quarrels are investigated closely, the problem becomes more difficult; for it is not merely a question of males fighting in the presence of a female, as is generally supposed to be the case, but on the contrary there is a complexity of strife which is bewildering—males attack females or vice versa; female fights with female; or a pair combine to drive away another pair, or even a solitary individual no matter of which sex. This complexity of strife makes against the view that the possession of a mate is the reason of the fighting. But an even stronger objection is to be found in the fact that males are hostile when no female is present—and hence we must seek elsewhere for the true explanation.

Now if the behaviour of a male be closely observed, it will be found that its pugnacious instinct gains or loses susceptibility according to the position which it happens to occupy—when its ground is trespassed upon, the impulse to fight is strong; but when it crosses the boundary it seems to lose all interest in the intruder. Moreover, in some species, the male rejoins the flock at intervals during the early part of the season and for a time leads a double existence, passing backwards and forwards between its territory and the neutral ground. Its behaviour under these circumstances affords some valuable evidence, for the bird displays little if any hostility when accompanying the flock, yet when it returns to the ground over which it exercises dominion, no male can approach without being attacked. The conclusion, therefore, seems to be inevitable, namely that the actual occupation of a territory is the condition under which the pugnacious nature of the male is rendered susceptible to appropriate stimulation.

CHAPTER IV

THE RELATION OF SONG TO THE TERRITORY

If we listen to the voices of the Waders as, in search of food, they follow the slowly ebbing tide, we shall notice that each species has a number of different cries, some of which are uttered frequently and others only occasionally. Not only so, but if we study the circumstances under which they are uttered, we shall in time learn to associate certain specific notes with certain definite situations.

The Curlew, when surprised, utters a cry with which most of us, I suppose, are familiar; but when with lowered head it drives away another individual from the feeding ground, it gives expression to its feelings by a low, raucous sound, which again is different from its cry when a Common Gull steals the arenicola that has been drawn out of the mud with such labour.

Thus we come to speak of "alarm notes," "notes of anger," "warning notes"—naming each according to the situations which normally accompany their utterance. And so, all species, or at least a large majority of them, have, in greater or lesser variety, cries and calls which are peculiar to certain seasons and certain situations; and since on many occasions we have indisputable evidence of the utility of the sound produced—as when, upon the alarm being given by one individual, the flock of Lapwing rises, or when, in response to a particular note of the parent, the nestling Blackcap ceases to call—so are we bound to infer that all the cries are, in one way or another, serviceable in furthering the life of the individual.

But besides these call-notes, birds produce special sounds during the season of reproduction—some by instrument, others by voice, others again by the aid of mechanical device. And not only is this the case, but many accompany their songs with peculiar flights, such as soaring to a great height, or circling, or floating in the air upon outstretched wings. These special sounds and special flights are those with which I now propose to deal, including under the heading "song" all sounds whether harsh or monotonous or beautiful, and whether vocally or otherwise produced; and I shall endeavour to show not only that they are related to the "territory," but that they contribute not a little to the successful attainment of reproduction.

The vocal productions are infinite in variety and combination. At the one extreme we have songs composed of a single note repeated slowly or rapidly as the case may be, whilst at the other we have the complex productions of the Warblers; and between these two extremes, notes and phrases are combined and recombined in ways innumerable. And just as there is a rich variety of combination, so there is a very wide variation in the purity and character of the notes—some are harsh, others melodious, some flute-like, others more of a whistle, and others again such as can only be likened to the notes of a stringed instrument. Hence in variety of phrase combination added to variety in the character of the note, there is a possibility of infinite modes of expression.

If, in the latter part of May, we take up a position at dawn in some osier bed, we listen to songs which have reached a high degree of specialisation, songs, moreover, which appeal to us on account of their beauty; if, on the other hand, we climb down the face of the sea cliff, we hear an entirely different class of songs—harsh, guttural, weird, monotonous sounds, which, appeal to us though they may, lack the music of the voices in the osier bed. And just as, in the osier bed, we can recognise each species by its voice, so we can distinguish the "cackle" of the Fulmar, the "croak" of the Guillemot, or the "grunt" of the Shag. In the osier bed, however, there is considerable variation in the song of different individuals of the same species, so much so that we can recognise this one from that; whereas on the cliff we cannot distinguish between the voices of different individuals. And the more highly developed the song, the greater the range of variation appears to be; but notwithstanding this—notwithstanding the fact that the pitch may differ, the phrase combination may differ, and the timbre may differ—the song remains nevertheless specific. So that the two principal features of "song," broadly speaking, are "diversity" and "specific character."

In contrast with the call-notes, the majority of which can be heard at all times of the year, the song is restricted as a rule to one season, and that one the season of reproduction. It is true, of course, that some birds sing during the autumn, and, if the climatic conditions are favourable, in the winter also, just as others betray, in the autumn, symptoms of emotional manifestation peculiar to the spring; but just as the manifestation of the latter is feeble and vestigial, so, too, does the song of the former lack the vigour and persistency which is characteristic of the spring. Again, in contrast with the call-notes, which are common alike to both sexes, song is confined to one sex—a peculiar property of the males.

Now all, I think, will agree that it must serve some biological purpose—this at least seems to be the conclusion to be drawn from the two outstanding features of "diversity" and "specific character"; and since the voices of different individuals of the same species vary, it has been suggested that, by creating a more effective pairing situation, it is serviceable in furthering the life of the individual. I do not propose at the moment to enquire whether this doctrine be true, but rather to direct attention to other ways in which the song may be useful.

Is the instinct susceptible to stimulation under all conditions during the season of reproduction, or only under some well-defined condition? This is the question to which we will first direct inquiry.

Song in its full development belongs, as we have seen, to the season of reproduction; it is heard at the dawn of the seasonal sexual process, and is the most conspicuous outward manifestation of the internal organic changes which ultimately lead to reproduction. These changes would appear, at first sight, to be the primary condition which renders the instinct susceptible to appropriate stimulation. But while this is true up to a point, in so far, that is to say, as organic changes are a necessary antecedent of all behaviour connected with the attainment of reproduction, closer acquaintance with the circumstances under which the instinct is allowed full play leads to the belief that they are not alone sufficient to account for the facts as observed. In order to arrive at a decision we must seek out the specific factors in the external environment with which "song" is definitely related.

Some birds cross whole continents on their way to the breeding grounds, others travel many miles, others again find suitable accommodation in a neighbouring parish—nearly all have a journey to perform, it may be short or it may be long. The flocks of Finches gradually decrease and we observe the males scattering in different directions in search of territories; we watch the summer migrants on their way—small parties halting for a few hours in the hedgerows and then continuing their journey, single individuals alighting on trees and bushes and resting there for a few minutes, and the constant passage of flocks of various dimensions at various altitudes; and we see Fieldfares, Redwings, and Bramblings slowly making their way from the south and the west to their homes in the far north. Occasionally we hear their song, not the emotional outburst customary at this season, but, except in isolated cases, a weak and tentative performance. Gätke speaks of the absence of song on the Island of Heligoland, and refers to the Whitethroat as one of the few migrants that enliven that desolate rock with their melody. On the other hand, many migrants that rest temporarily on the Isle of May sing vigorously.[5] But on the whole there is, I think, no question that the male whilst travelling to its breeding grounds, and, even after its arrival, whilst in search of a territory, sings but little—and that little lacks the persistency characteristic of the period of sexual activity. Yet, when a suitable territory is eventually secured, the nature of the bird seems to change; for, instead of being silent and retiring, as if aware of some end not fully attained, it not only makes itself conspicuous but advertises its presence by a song uttered with such perseverance as to suggest that that end is at length attained. Hence, in a general way, the instinct of song seems to be related to the establishment of a territory.

Now the subsequent course of behaviour tends to confirm this view. We have already had occasion to refer to the fact that the males of some species desert their territories temporarily and join together on ground which is regarded by the birds that associate there as neutral, and that they do so not merely for the purpose of securing food but because they derive some special pleasure from the act of association, and we shall find that the altered behaviour of the male when it leaves its territory to seek food or to join the flock is an important point for us just now.

Buntings desert their territories temporarily and collect in flocks on the newly sown fields of grain. Some of the males are single, others are paired, and accompanied, it may be, by their mates; they wander over the ground in search of food, uttering their call-notes from time to time, or, settling upon the hedges and trees surrounding the field, rest there and preen their feathers. But even though a male may be surrounded by other males, even though it may occupy a position where it is conspicuous to all around, even though, that is to say, it is apparently in contact with just those stimulating circumstances which will evoke a response when it returns to its territory, yet it makes no attempt to sing.

Lapwings, when they resort to the neutral ground, run this way and that in full enjoyment of one another's companionship, behaving as they do when they flock in autumn and winter. Specific emotional manifestation is, however, absent, and their actions seem to be in nowise affected by the powerful impulse which only a few minutes previously determined their conduct, for of the characteristic flight with its accompanying cry there is no sign.

Early in the season Turtle Doves often collect from the surrounding country at certain spots where their favourite food is abundant. The croak of this Dove—its true song—is a familiar sound during the summer, but in addition the bird has a sexual note characteristic of the race. I watched a flock of upwards of one hundred on some derelict ground approximately eight acres in extent. Here, in May, the birds were attracted by the seeds of Stellaria media which was growing in profusion. After 5 A.M. there was continuous traffic between this piece of ground and the surrounding neighbourhood, a constant arrival and departure of single individuals or pairs; and, as they fed, the sexual note could be heard in all directions. Now some of the males occupied territories close at hand, and one could watch their passage to and fro; yet in no single instance did I hear the true song uttered on the feeding ground, although the moment a male returned to its territory its monotonous croak could be heard, uttered moreover with that persistence which is so marked a feature of all song or of the sounds that correspond to it.

Thus it will be seen that, even after the internal organic changes have taken place, the instinct of song is not susceptible to stimulation at all times and under all circumstances, but only at certain specified times and under special circumstances which can be observed to correspond with the occupation of the territory.

In many species each male singles out within its territory some prominent position to which it resorts with growing frequency. This position is an important feature of the territory, and exercises a dominating influence on the life of the bird. I have referred to it as the "headquarters," and it may be a solitary tree or bush, an outstanding mound or mole hillock, a gatepost or a railing—anything in fact that supplies a convenient resting place so long as it fulfils one condition, namely that the bird when it is there is conspicuous. It need not, however, be a tree or a mound or indeed anything upon which the bird can perch, for there is reason to think that the soaring flight undertaken at this season by so many males, since it is generally accompanied by the specific sexual sound, answers the same purpose as the topmost branch of a tree.

Now there is nothing in the external environment to which the song is more definitely related than to the "headquarters"—this at least is the conclusion to be drawn from the behaviour, and I will indicate the sort of evidence upon which such conclusion is based. There is, first of all, the persistency with which the male resorts to the same tree, even to the same branch, and, as it seems, solely for the purpose of advertisement. We know by experience the approximate routine of the male's behaviour; we know where to seek it, where to hear it, and when once we have discovered its headquarters, we know that there it will sing day after day for weeks or it may be for months together—perhaps the most striking feature of its behaviour at this season. Next, we find that other trees, though made use of, are not made use of to a similar extent for the purpose of song. The area occupied varies much according to the nature of the environment; it is sometimes extensive, and seldom less than half an acre or so in extent; but in most instances it contains plenty of trees and bushes which could, one would imagine, serve the purpose of a "headquarters" just as well as the particular one selected, and yet the bird, when there, betrays no inclination to sing at all comparable with that which can be observed when it occupies its accustomed perch. Further evidence is afforded in the behaviour of those species that make temporary excursions from their territories. The male, on its return, flies as a rule direct to its special tree and sings. Sometimes, however, it settles upon the ground, not unfrequently accompanied by the female, and while there remains silent; but presently rising from the ground and deserting its mate, it flies to the headquarters and sings. Again, nearly every male at one time or another in the course of the season is aroused to action by the intrusion of a rival. The emotional tone of the owner of the territory is then raised, and the intruder is pursued and attacked; but this alone is not sufficient, it seems as if the chain of instinctive activities, when once aroused by appropriate stimulation, must pursue its course to the end—and the end in such a case is only reached and complete satisfaction only gained when the bird has not merely returned to his "headquarters" but has given vocal expression to his emotion. Finally, we must bear in mind these two facts, that the "headquarters" is occupied solely by the male—it forms no part of the life of the female—and that it is the male only that sings.

Many such subtle incidents of behaviour as the foregoing can be perceived but not readily described, and trifling though they may seem to be in themselves, yet in the aggregate they yield full assurance of a close relationship.

The distant song of a male, or the presence of an intruding male, have also stimulating effects, though in somewhat different ways. The former evokes the normal reply, that is to say the bird, if silent, is liable to utter a corresponding reply; the latter arouses hostility into which is infused much feeling tone, the bird sings hurriedly while in pursuit of its rival, and, which is more remarkable still, even in the midst of an encounter. Both the normal reply and the emotional song must be similar in origin—different aspects of the same situation—and both are clearly related to the other male.

The arrival of a female may also be followed by an emotional outburst which can be heard at intervals for some days; on the other hand, the song may continue as before or, for a time, entirely cease.

To take the emotional outburst first. This would appear to be susceptible of explanation on the hypothesis that the voice contributes to a more effective pairing situation; an hypothesis which admittedly, at first sight, gains some support from the fact that a second or a third male is frequently present. But, in truth, the presence of a second male makes the situation, so far as the relationship between the song and the female is concerned, all the more perplexing; for, as we have already seen, the instinct of pugnacity, when aroused by the appearance of an intruder, is also liable to be accompanied by a similarly extravagant song. On each occasion the vocal effort is infused with much feeling tone, and it would be impossible to point to any one feature which is peculiar to only one occasion. The question therefore arises as to whether the emotional outburst which we are attributing to the arrival of a female may not after all be due to the presence of an intruding male. It may be so. But although I can recall no single instance in which the presence of an intruder could be definitely excluded, yet I should hesitate to base upon this any broad generalisation.

When the normal course of the song is not interrupted by the arrival of a female, when, that is to say, the male still pursues the routine to which he has all along been accustomed, and still sings at stated intervals in stated places with a voice that betrays no heightened emotional tone, even though the song may convey some meaning to the delicate perceptual powers of the female, we have nothing to lay hold upon which can be construed as an indication of direct relationship between the song and the presence of the female.

The partial or complete suspension of the song after pairing has taken place is the most interesting, as it is the most noticeable, feature. Not that it is by any means universal—if it were so, some of the difficulties that beset the path of interpretation would be removed, but it is sufficiently widespread to demand explanation. In nearly every case it is, however, only temporary, the period during which the male is silent varying from a few days to a few weeks. The male Grasshopper-Warbler, when it first reaches us, sings persistently, but when it is joined by a female a change becomes apparent; instead of the incessant trill, there are spasmodic outbursts of short duration, and in the course of a few days the bird lapses into a silence which may be broken for a short while at dawn, or late in the evening, but is often complete. More striking still is the change in the case of the Marsh-Warbler, and the sudden deterioration, or even suspension, of strains so beautiful and so varied, at a moment, too, when it might least be expected, at once arrests the attention. The Reed-Warbler that had its headquarters in a willow sang vigorously from the middle of May until a female arrived on the 20th June, when its voice was hushed, except for occasional outbursts which lacked force and were of short duration. When the Wood-Warbler secures a territory it repeats its sibilant trill with unwearying zeal, yet no sooner does a mate appear than its emotion is manifested in other directions. The Reed-Bunting is vociferous during February and March; but when a female arrives, periods of silence are frequent and the instinct of the bird becomes progressively less susceptible to stimulation. After the manner of the race the male makes temporary excursions from its territory accompanied by his mate, and it is noteworthy that when he returns and she is absent he sings, but that the moment she joins him, or even comes into sight, he is silent. In fact, in greater or less degree, a change is noticeable in the song of many resident and migratory species under similar circumstances, a deterioration so marked that we learn by experience to regard it as a certain indication of the arrival of a mate.

Thus it becomes clear that there are certain specific factors in the external environment with which the instinct can be definitely related, and in the order of their importance they are (1) the territory as a whole; (2) the headquarters; (3) an intruding male; (4) the female.

To what extent are these relationships interrelated? Are they all mutually dependent upon one another, or is there one which conditions the remainder?

In the first place it is evident that if a male were not to establish a territory, no opportunity would be afforded for making use of any special post or for acquiring a habit in relation to it, and so without further consideration we may say that the connection between the song and the headquarters, whatever it may be, is primarily dependent upon the establishment of a territory.

Next, we have the fact that the distant voice, or still more so the presence, of another male has an exciting influence and evokes a corresponding reply. Here we have a direct relationship, and one which at first sight appears to be exclusive of cross-correlation. But is it really so; does no circumstance arise under which even the proximity of a rival fails to evoke response? The reply is not doubtful. Such a circumstance does arise—when a male for one reason or another passes outside the limits of its accustomed area. This aspect of behaviour has already been fully discussed in connection with the question of hostility, and everyone, I imagine, must by now be pretty well familiar with the facts. However, it does not often happen that we are given such an aid to interpretation as is vouchsafed to us in the altered behaviour of the male when it joins the flock, and if, as I believe, song and hostility are intimately associated, forming part of an inter-related whole which, for biological interpretation, has, as its end, the attainment of reproduction, it is not surprising that circumstances which lead to the modification of the one should likewise affect the other; I offer no apology, therefore, for adverting to this aspect of behaviour once again.

Now a male may leave its territory for three reasons—to pursue an intruder, to join the flock on neutral ground, or to find the necessary means of subsistence on other feeding grounds. On each of these occasions it hears the song of, and is in close contact with, other males; and if the relationship of which we are speaking be really exclusive of cross-correlation, its instinct ought to respond with the customary freedom. But what happens? A male pursues its rival, betraying much emotion and singing extravagantly, until the boundary is passed, when emotion subsides and it is silent; or, it flies to the flock on neutral ground, and, although surrounded by the very males that a short time previously evoked response, is there unresponsive; or again, it goes in search of food and collects with other males bent on a similar errand, and in presence of what we know would be an exciting influence under other circumstances, it nevertheless remains silent. Hence the relationship between the song and a male rival seems, as in the case of the headquarters, to depend in the first instance upon the occupation of a territory.

So that the relationship between the song and the territory as a whole is clearly of a different order from that which obtains between the song and the headquarters, or the song and a male rival; for the first, as far as can be judged by observation, is exclusive of, whilst the second and the third involve, cross-correlation. How are these facts to be explained? We have already seen that it belongs to the nature of the male during the season of reproduction to establish itself in a definite place, and this action is just as much a part of its hereditary nature as the building of the nest is of that of the female, and it is just as necessary for the successful attainment of reproduction. What exactly the stimulus is to this mode of behaviour we do not know; we can go no further back than the internal organic changes which are known to occur and which we assume, not without some reason, are responsible for its initiation. Granting, then, that there is this congenital disposition, what relation does it bear to the song? Without a doubt the song is likewise founded upon a congenital basis; it is truly instinctive, and as such requires appropriate stimulation; furthermore the male sings only when in occupation of its territory. Having regard to these two facts we might say that the territory is the stimulus to the song. But this can scarcely be a true interpretation, for inasmuch as the stimulus would be relatively constant, a relatively constant response ought to follow, and even a slight acquaintance with the daily round of behaviour will furnish plenty of evidence to the contrary, seeing that the song, though persistent, is never continuous—in fact there are long periods of silence during the daytime, and only in the morning and the evening does the male become really vociferous. What then is the stimulus? Through awareness of something in the environment the male responds to stimulation, and the only reply we can give is that the headquarters, or a distant song, or the proximity of another male—with all of which, as we have seen, the instinct is definitely related—are the specific factors which normally evoke response—and experience teaches us that the periods of quiescence are just those when life is at its lowest ebb and these stimulating factors less in evidence. Bearing this in mind, bearing in mind the fact that when a male joins the flock or crosses the boundary its instinct ceases to respond, bearing in mind, that is to say, that there is evidence of relationship between these specific factors and the song only when the territory is actually occupied, the conclusion seems inevitable that we have here the determining condition which renders the instinct susceptible to appropriate stimulation.

There remains the female. I place her last in order of importance, not because I regard her influence as of small consequence, but because the evidence is of a varied and complex kind, so much so that it is difficult to ascertain by observation just how far she is a situational item. It will be remembered that the only direct evidence we had of such influence was a deterioration or, in some instances, a complete cessation of vocal manifestation. Clearly then we are confronted with a relationship of a different kind from that which we have been discussing; for not only is anything in the nature of stimulation absent, but, and this is a remarkable fact, the other items in the environment which formerly evoked response no longer do so in quite the same way. Is there any awareness on the part of the male of the relation between his voice and the mate that is to be, or is it merely that as the sexual situation increases in complexity some inhibiting influence comes into play? These are questions which lead up to difficult problems. But it is no part of my task to discuss the psychological aspect of the behaviour; my purpose is merely to show that the situation on the arrival of a female undergoes marked modification, that the instinct of the male is then less susceptible to stimulation, and that the factors in the external environment which formerly elicited response become relatively neutral.

Hence the appearance of the female on the scene marks the opening of a new stage in the life-history of the male, and, to judge by the course of events, it would seem as if the song with its network of relationships had now served its main biological purpose.

And now, what is the purpose, and what the origin, of song? Is it, as some naturalists have conceived, a means of raising the emotional tone of the female, of creating a more effective pairing situation, and so of removing a barrier to the successful discharge of the sexual function; or, is the emphasis here too much upon the emotional, too little upon the strictly utilitarian, aspect? All, I think, will agree that it must serve some biological purpose, and the position we have so far reached is that the determining condition of its manifestation is not merely the establishment, but the actual occupation of a territory, and that there are no factors in the external environment which can evoke response in the absence of such condition. This being so, the further questions arise as to whether it contributes towards the attainment of the end for which the whole territorial system has been built up, and what precisely is the way in which it does so.

Everyone knows that in the spring the shyest of birds no longer practise the art of concealment. The Curlew soars to a great height, and upon outstretched wings hovers in the air whilst uttering its plaintive wail; the cock Grouse, as if dissatisfied with its "crowing," springs into the air and becomes a conspicuous object of the moor; the wary Redshank, poised on flickering wings, forgets its mournful alarm cry, and finds again its melodious song; and even the secretive Grasshopper-Warbler crawls out of the midst of the thicket in order to "reel," just as, for a similar reason, Savi's Warbler climbs to the top of a tall reed. In fact the males of most species, when they are finally established on the breeding grounds, make themselves as conspicuous as possible by sight and by sound. And since the sounds produced by no two species are exactly alike, the females are able to recognise their prospective mates, and the males that are still in search of ground have ample warning if that upon which they are treading is already occupied. So that you see, from the remarkable development of the vocal powers in the male, there follow two important results—"recognition" and "warning."

We here turn from song as the expression of an instinctive disposition, and the question of what calls forth this expression, to the impression produced by the song on the hearer.

Most birds have a call-note or a number of call-notes, which, generally speaking, are specifically distinct. But to the human ear they are not always so, perhaps because our power of hearing is less sensitive than that of a bird, and unable to appreciate delicate differences of tone. Be this as it may, however, the fact remains that we often find it difficult, and in not a few cases impossible, to recognise a bird merely by its call. The plaintive notes of the Willow-Warbler and of the Chiffchaff are to our ears very closely akin, so, too, are those of the Marsh-Warbler and of the Reed-Warbler, and there is a great resemblance between the hissing sound produced by the two Whitethroats. In Co. Donegal I have been deceived by the spring-call of the Chaffinch which, owing possibly to the humidity of the atmosphere, is, there, almost indistinguishable from the corresponding note of the Greenfinch. The Yellow Bunting and the Cirl Bunting frequently make use of a similar note, so do the Curlew and the Whimbrel. In fact, numberless instances could be quoted in which notes appear to us identical, and, as a rule, the more closely related the species, the more difficult it becomes to distinguish the sounds—alike in plumage, alike in behaviour, alike in emotional manifestation, it would be surprising if they were not alike in voice. But the moment we pass from the call-notes to a consideration of the songs we are faced with a very remarkable fact, for not only are these readily distinguished, but in many cases they bear no resemblance in any single characteristic. What could be more unlike than the songs of the Willow-Warbler and of the Chiffchaff, of the Marsh-Warbler and the Reed-Warbler, or of the Yellow Bunting and the Cirl Bunting?

Now when different individuals collect in flocks at certain seasons, they assist one another in finding food, and afford mutual protection by giving timely warning of the approach of a common enemy, and the gregarious instinct is thus of great advantage to the species; but no matter how powerful the impulse to flock might be, if there were no adequate means of communication, the different units would frequently fail to discover their neighbours. Here the specific cries and calls come into play, enabling them as they move about in search of food, or change their feeding grounds, or whilst they are on migration, to keep constantly in touch with one another; and hence one purpose that these call-notes serve is that of recognition. Moreover, they convey their meaning to individuals of other species and are acted upon, and are thus in every sense socially serviceable; but on the other hand, whilst there is much evidence to show that the song is of great individual value, there is none to show that it is in any like manner of direct advantage to the community.

If, then, there is in the call-notes an adequate means of communication and of recognition, why do I suggest that the song has also been evolved primarily for the purpose of recognition?

What, first of all, are the conditions in the life behaviour during the season of reproduction that make the intervention of the voice a consideration of such importance? The general result of our investigation might be summed up thus: we found that the male inherits a disposition to secure a territory, that at the proper season this disposition comes into functional activity and leads to its establishment in a definite place, and that it cannot search for a mate because its freedom of action in this respect is forbidden by law; that the female inherits no such disposition, that she is free to move from place to place, free to satisfy her predominant inclination, and to seek a mate where she wills; and, since the appropriate organic condition which leads to pairing must coincide with appropriate conditions in the environment, that the union of the sexes must be accomplished without undue delay. Furthermore we found that a territory is essential if the offspring are to be successfully reared; that, since the available breeding ground is limited, competition for it is severe, and that the male is precluded from leaving the ground which he has selected, and is obliged, in order to secure a mate, to make himself conspicuous. That was our general result. Now there are two ways by which the male can make himself conspicuous—by occupying such a position that he can be readily seen, or by producing some special sound which will be audible to the female and direct her to the spot. The former, by itself, is insufficient; in the dim light of the early dawn, when life is at its highest, and mating proceeds apace, what aid would it be for a male to perch on the topmost branch of a tree, how slender a guide in the depth of the forest? But whether in the twilight or in the dark, in the thicket or the jungle, on the mountain or on the moor, the voice can always be heard—and the voice is the principal medium through which the sexes are brought into contact.

Well now, we come back to the question, why, if all species have a serviceable recognition call, that call should not be sufficient for the purpose, just as, without a doubt, it is adequate for all purposes at other seasons? The answer is, I think, clear. The recognition call is not confined to one sex, nor only to breeding birds; it is the common property of all the individuals of the species, and if the female were to rely upon it as a guide she might at one moment pursue another female, at another a non-breeding male; she might even be guided to a paired female or to a paired male, and time would be wasted and much confusion arise. So that no matter how much a male might advertise himself by cries and calls which were common alike to all the individuals of the species, it would not assist the biological end which we have in view. Something else is therefore required to meet the peculiar circumstances, some special sound bearing a definite meaning by which the female can recognise, amongst the host of individuals of no consequence to her, just those particular males in a position to breed and ready to receive mates. Hence the vocal powers, the power of producing sounds instrumentally, and the power of flight, have been organised to subserve the biological end of "recognition."

And this view is strengthened, it seems to me, by the erratic behaviour of certain species, more particularly by one remarkable case, the case of the Cuckoo. The male, after having established himself, utters his call persistently from the day of arrival until approximately the middle of June; but, in contrast with the large majority of species, the female has a characteristic call which she, too, utters at frequent intervals. The female is polyandrous and has a sphere of influence embracing the territories of a number of males; she wanders from place to place, is often silent, and not unfrequently is engaged in dealing with her egg or in searching for a nest in which to deposit it, and therefore she is not always in touch with a male, still less with any particular one. Now there is much evidence to show that the discharge of the sexual function amongst birds is subject to control, and that this control operates through the female—through her physiological state becoming susceptible to stimulation only at certain periods. So that we have these considerations, that the female is polyandrous, that she has a territory distinct from that of the male, and that her sexual impulse is periodical; and the further consideration that the impulse, since it is periodical, is of limited duration and must receive immediate satisfaction. Such being the circumstances of the case, would the voice of the male serve to insure the union of the sexes at the appropriate moment? Well, the fact that she is polyandrous implies that every male in her sphere of influence is not always capable of satisfying her sexual instinct. Is, then, the male's call an indication of his readiness to yield to stimulation? Without a doubt it is an index of the general physiological state which generates the sexual impulse, without a doubt it denotes a general preparedness to breed, but there is no evidence to show that it denotes the degree of ardour of the male at any particular moment, and much that proves the contrary. So that only by the female producing some special sound which will attract the males that are eager and bring them rapidly to the spot where she happens to be, only thus is it possible to insure the consummation of the sexual act. This, it seems to me, is the purpose of the peculiar call of the female—a call which, so far as biological interpretation is concerned, is just as much a song as the melody of the Marsh-Warbler—and its interest for us just now lies in this, that here we have a special case in which the sexes have separate territories, the female is polyandrous, and the voice of the male is not sufficient by itself to bring to pass the union of the sexes; and in which, consequently, if the purpose of song be that of recognition, we should expect to find, as we do find, that the female had a distinct and penetrating call.

We now come to the question of "warning," by no means the least important purpose of song. I pointed out that one of the chief differences between the call-notes and the song was that the former were socially serviceable, whereas the latter was only serviceable to certain individuals; and in making this statement, I had in mind the direct benefits to the community which proceeded from an appreciation of sounds having a mutually beneficial meaning, not the indirect, though none the less beneficial, consequences to the species as a whole. Biologically considered, song, if it acts as a warning and thereby leads in one way or another to more complete success in the rearing of offspring, may be spoken of as socially serviceable; but it is legitimate to draw a distinction between the prospective value of remote relationships which we can foresee, and the mutual assistance which the individuals of a community derive from their close association.

If there were always sufficient breeding ground to support the offspring of all the individuals of each species, if the individuals were always so distributed that there was no possibility of overcrowding in any particular area, and if the conditions of existence of different species were so widely divergent that the presence of this one in no way affected the interests of that, no opportunity would be afforded for the development of so complex a system as is involved in the "territory" and all that appertains to it. But the available breeding ground is by no means unlimited. The supply of food, which is a determining factor in the environment, is always fluctuating according to the climate and according to the changes in the earth's surface; and so the distribution of the bird population in any given area, though it may be suitably adjusted for one year or even for a period of years, is bound in the course of time to require readjustment. Now there cannot be readjustment without competition, nor competition without combat. But the appeal to physical force is only a means to an end, and, since no male can endure incessant warfare and the perpetual strain of always being on the alert, without experiencing such physical exhaustion as might affect his power of reproduction, its direct effect upon the combatants cannot be otherwise than harmful—in fact it is a necessary evil which for the good of the species must be kept strictly within bounds. Bearing in mind, then, these two facts, namely that the distribution of the males is never stable and that overmuch fighting may defeat the end in view, we can appreciate the importance of any factor which will lead to a more uniform distribution and at the same time insure security by peaceable means.

The proximate end of the male's behaviour is isolation—how is it to be obtained? If, after having occupied a territory, the bird were to remain silent, it would run the risk of being approached by rivals; if, on the other hand, it were merely to utter the recognition call of the species, it would but attract them. In neither case would the end in view be furthered, and isolation would solely depend upon alertness and the capacity to eject intruders. Supposing, however, that the song, just as it serves to attract the females, serves to repel other males, a new element is introduced deserving of recognition; for those males that had established themselves would not only be spared the necessity of many a conflict, but they would be spared also the necessity of constant watchfulness, and so, being free to pursue their normal routine—to seek food, to rest, and, if migrants, to recover from the fatigue of the journey, they would be better fitted to withstand the strain of reproduction; and those that were still seeking isolation in an appropriate environment, instead of settling first here and then there only to find themselves forestalled, would avoid and pass by positions that were occupied, establishing themselves without loss of time in those that were vacant. Without the aid of something beyond mere physical encounter to regulate dispersal, it is difficult to imagine how in the short time at disposal anything approaching uniformity of distribution could be obtained. Hence, both in the direction of limiting combat, of insuring accommodation for the maximum number of pairs in the minimum area, and of conserving energy, the song, by conveying a warning, plays an important part in the whole scheme.

And if this be so, if the song repels instead of attracting, it follows that the more distinct the sounds, the less likelihood will there be of confusion; for supposing that different species were to develop similar songs, whole areas might be left without their complement of pairs just because this male mistook the voice of that, and avoided it when there was no necessity for doing so. So that just as from the point of view of "recognition" each female must be able to distinguish the voice of its own kind, so likewise the warning can only be adequate providing that the sounds are specifically distinct. A point, however, arises here in regard to closely related forms. Some species require similar food and live under similar conditions of existence; they meet in competition and fight with one another; and, if they did not do so, the food-supply of a given area would be inadequate to support the offspring of all the pairs inhabiting that area. Generally speaking, the more closely related the forms happen to be, the more severe the competition tends to become; and it may be argued that in such cases a similar song would contribute to more effective distribution and in some measure provide against the necessity of physical encounter; that, in fact, it would stand in like relation to the success of all the individuals concerned, as does the song to the individuals of the same species. But we must bear in mind that the primary purpose of song is to direct the females to those males that are in a position to breed; and to risk the possibility of prompt recognition in order that the males of closely related species should fight the less, would be to sacrifice that which is indispensable for a more remote and less important advantage.

What meaning does the song convey to a male that is unestablished? Does the bird recognise that it is forestalled; does it foresee and fear the possibility of a conflict, and conclude that the attempt to settle is not worth while? I do not imagine that it thinks about it at all. How then does the warning warn? We will endeavour to answer this question, but, in order to do so, we must review the stages by which a territory is secured.

We take as our starting point the internal organic changes which are known to occur. These changes are correlated with other changes, manifested by a conspicuous alteration in behaviour—to wit, the disappearance of sociability and its replacement by isolation. Having found a station which meets the requirements of its racial characteristics, the male establishes itself for a season, becomes vociferous, displays hostility towards others of its kind, and in due course is discovered by a female. The whole is thus an inter-related whole, a chain of activities which follow one another in ordered sequence. Now we have seen that it is neither pugnacious nor vociferous until the territory is actually occupied; we have seen that the fact of occupation is the condition under which the instincts of pugnacity and of song are rendered susceptible to appropriate stimulation; we have discussed the nature of the stimulus in each case, and we wish to know the sort of meaning that the song conveys to an individual which is still in the preliminary stage of seeking a station. In sequential order we have the following: (1) internal organic changes which lead to isolation, (2) the appropriate environment which gives rise to an impulse to remain in it, (3) the occupation of a territory which is the condition under which the instincts are rendered susceptible to stimulation, (4) the various stimuli. Each is dependent upon that which precedes it, and no part can be subtracted without failure of the biological end in view, neither can the different stages be combined in different order. So that, in considering the significance of song to an unestablished male, we are dealing with the situation at a point at which all the latent activities have not been fully felt, for all that so far has occurred is the change from sociability to isolation determined by internal organic changes. The bird has not established a territory because it has not come into contact with the appropriate environment, and it is not pugnacious because the condition which renders its instinct susceptible is absent; and so, as it wanders from place to place and hears the voices of males here or males there, it merely behaves in accordance with that part of its nature which predominates just at that particular moment—the impulse to avoid them.

But given the appropriate environment, given, that is to say, just that combination of circumstances which might bring into functional activity all the latent instincts of the intruder, and no matter how vociferous the occupant of a territory might be, it would not be preserved from molestation. The advantage of the song, biologically considered, is then this, that it will often prove just sufficient to preclude males in search of isolation from coming into contact with the environmental conditions adequate to supply the stimulus to their latent activities and to convert them into rivals.

If this interpretation be correct, if we are right in attributing the withdrawal solely to the fact that the first stage only in the relational series has been reached, it follows that the effect of song upon males that have reached subsequent stages in that series must be of a very different kind. We have dealt with the male when in the preliminary stage of seeking isolation, we must deal with it now when eventually it occupies a territory. How does it behave when it hears, as it is bound to do, the voices of rivals in its neighbourhood? You may remember that some allusion was made to the fact that an outburst of song from one individual was followed, not unfrequently, by a similar outburst on the part of other individuals in the immediate locality. For example, silence may reign in the reed-bed except for an occasional note of the Reed-Warbler or Sedge-Warbler. Suddenly, however, a dispute arises between two individuals, accompanied by a violent outburst of song, and forthwith other males in the vicinity begin to sing excitedly and continue doing so for some minutes in a strangely vigorous manner, the tumult of voices affording a striking contrast to the previous silence. Spasmodic outbursts of this kind, stimulated by an isolated utterance, are by no means uncommon. But not only does song stimulate song; under certain conditions it has the still more remarkable effect of arousing hostility. The boundary that separates two adjoining territories is by no means a definite line, but rather a fluid area wandered over by this owner at one moment, by that at another. Now so long as the bird is silent while in this area, the probability is that it will escape detection and remain unmolested; let it however sing—it often does so—and it will not merely be approached but attacked, and consequently this area is the scene of much strife. The point to be noticed here is that the song brings about no withdrawal; it elicits a response, attracts instead of repelling, and, in short, arouses the impulse that is always predominant in the nature of the male when eventually it occupies a territory—the impulse of self-assertiveness. Therefore it seems clear that the different stages in the process of reproduction mark the appearance of different conditions, each of which renders some new impulse susceptible to stimulation, and that the significance of song depends upon the stage which happens to have been reached. Hence when we speak of song acting as a "warning," we do not mean that it arouses any sensation of fear; it is but a stimulus to that part of the inherited nature of the hearer which predominates at the moment.

Are we then justified in the use of such terms as "warning," "significance," or even "meaning," when it is but a matter of stimulus and response? In what does the impulse to avoid other males consist? There is no reason to suppose that there is any sensation of fear in the first stage, and the course of behaviour demonstrates that there is none in the later stages. But it is difficult to conceive of an impulse which has, as its end, the isolation of the individual from members of its own sex and kind, without some feeling-tone, the reverse of pleasurable, entering into the situation; just as it is difficult to believe that the female experiences no pleasurable sensation when she hears the voice of the male that directs her search. So that the song may be actually repellent in the one case and attractive in the other; and it is none the less repellent when, as in the later stages, it attracts a neighbouring male, for the attraction is then of a different order, determined by the presence of the condition which renders the pugnacious nature susceptible and leads to attack. In a sense, therefore, we can speak of "meaning"—though not perhaps of "significance"—and of "warning," when we refer to the prospective value of the behaviour.

So much for the purpose of "song"; there still remains the more difficult question—the question of origin. Let me make clear what I mean by origin. As we have already seen, there is infinite diversity in the sexual voice of different species; some are harsh and others monotonous, and some strike the imagination by their novelty whilst others are melodious; and to the naturalist each, in its particular way and in a particular degree, probably makes some appeal according to the associations that it arouses. But just why a Marsh-Warbler is gifted with a voice that is so beautiful and varied, whilst the Grasshopper-Warbler must perforce remain content with a monotonous trill; just why the tail feathers of the Snipe have developed into an instrument, whilst the Pied Woodpecker has developed muscles which enable it to make use of a decayed branch as an instrument—we know no more than we do of the nature of the forces which lead the Reed-Warbler to weave its nest to reeds, or the caterpillar of the Elephant Hawk Moth to assume so peculiar an attitude when disturbed. When therefore I speak of the origin, I do not refer to the mode of origin of variation; I take for granted that variations somehow arise, and I seek to ascertain whether there is anything in the phenomena which we have explored which might reasonably be held to determine the survival of this one in preference to that.

When we reflect upon the problem of song and consider the numerous and diverse forms in which it is manifested, we are apt to draw a comparison between the sounds we hear and those produced by musical instruments, and hence to conclude that each bird is gifted with a special instrument in virtue of which it produces its characteristic melody. But there is a very remarkable phenomenon connected with the singing of birds which shows that this is really not the case—I mean the phenomenon of imitation. There are plenty of good imitators amongst our native species, and the power of imitation is not the exclusive property of those which have reached a high degree of vocal development, nor, for the matter of that, of song-birds at all. Even the Jay, than which few birds have a more raucous voice, that "hoots" like the Wood-Owl, or copies the sounds produced by the tail feathers of the Snipe, will occasionally imitate the most melodious strains of some other species; and the Red-backed Shrike, whose sexual call is principally a few harsh notes rapidly repeated, bursts at times into perfect imitations of the song of the Swallow, Linnet, or Chaffinch. Nevertheless it is amongst such typical songsters as the Warblers that we find the greatest volume of imitation, and no limit seems to be placed upon their capacity. The Marsh-Warbler can utter the call of the Green Woodpecker, or sing as the Nightingale does, with as much facility as it sings its own song; and the Blackcap is well-nigh as proficient in copying the cries and melodies of surrounding species—and so, if it were necessary, we might proceed to add to the list.

These examples demonstrate that different songs are not represented by a corresponding number of different physiological contrivances; for if the difference were really attributable to some structural peculiarity, then the range of sounds embraced in the call-notes and the sexual call of any given species, must be the measure of the capacity of its instrument; and no matter how great its power of imitation may be, it follows that it will only be capable of copying those sounds which fall within that range. There is plenty of evidence to show that the power of imitation is almost unlimited, at all events that it is not confined within such narrow limits as are here demanded. Hence it seems clear that the diversity of song is not to be sought in structure, but in some innate capacity to play one tune in preference to another; and if this be so, and if out of the same instrument, which has been primarily evolved to further the biological end of intercommunication, all manner of diverse sounds can be made to proceed, the problem of the origin of song is to that extent simplified.

We must next inquire into the nature of song, and endeavour to ascertain whether all the individuals of a species are alike proficient, or, failing this, whether there is any quality which can be observed to be constant under all conditions. I watch the Reed-Buntings in a marsh and find that there are three males occupying adjoining territories. Two of them are fully mature and their plumage is bright: that is to say the crown is black, the collar and breast are white, the flanks are dull white spotted with black, and the mantle is reddish-brown. The third is immature: the crown, instead of being black, is suffused with brown; the collar, instead of being white, is mottled with brown; and the flanks are more heavily streaked with brown. These three birds take up their positions in February, and, as is their wont, sing incessantly each day at daybreak. The song of the first two is normal, including the usual number of phrases which flow in no definite sequence, but are combined and recombined in different order, and the tone is pure; that of the third, the immature bird, is, however, very different; for just as in comparison its plumage is dull, so the phrases of its song are limited and reiterated with great monotony, the tone is impure, and the whole performance is dull and to our ears unmusical. I watch them from February to June, and observe the order in which they are mated—first a mature male; next, after a short interval, the immature male; and finally, after a still longer interval, the remaining bird gets a mate. As the season advances, still keeping watch on the development of the plumage and of the voice of the immature male, I observe that no very definite change takes place—that the colours remain dull, that there is a conspicuous absence in the song of certain phrases, and that the notes lack purity of tone.

If now, instead of Reed-Buntings in a marsh, I watch Yellow Buntings on a furze-covered common, I find that, establishing themselves early in February, they sing persistently, and in a few weeks are paired. But what arrests my attention more particularly is the quality of the song; for although the voice is unmistakably the voice of the Yellow Bunting, yet it is incomplete and lacks the variety of phrases and musical notation which we customarily associate with the bird. Nevertheless, as the season advances, there is a progressive development in both these directions, and by the end of March or the beginning of April the song possesses all those qualities which appeal to us so forcibly.

There is one other fact to which attention must be drawn—the variation in the song of the same species in different districts. As an illustration let us take the case of the Chaffinch. In Worcestershire the bird sings what I imagine to be a normal song—the notes are clear and the phrases are distinct and combined in numerous ways. With the notes fresh in mind I leave them and go to the west of Donegal, where I am at once conscious of a difference; not a subtle difference that perplexes the mind and is difficult to trace, but a change so remarkable that one is conscious of a passing doubt as to whether after all the voice is the voice of the Chaffinch; the song is pitched in a lower key, certain phrases are absent, the notes lack tone and are sometimes even harsh, and the bird seems wholly incapable of reaching the higher notes to which I am accustomed.

Now the immature Reed-Bunting, though to our ears its song is but a poor representation of that of the adult, gains a mate; the Yellow Bunting pairs, and the discharge of the sexual function may even have taken place before its voice attains what we judge to be its full development; and there are no grounds for supposing that the Donegal Chaffinch, with its less musical notes, has on that account any the less chance of procreating its kind—facts which demonstrate that the biological value of song is neither to be sought in the purity of tone, nor in the variety and combination of phrases, nor, indeed, in any of those qualities by which the human voice gains or loses merit, and which leave us with no alternative but to dismiss from our minds all æsthetic considerations in the attempt to estimate its true significance.

What, then, determines its value? Are there any qualities which, whether the bird is mature or immature, whether it is untrained or has acquired fuller expression by practice, whether it inhabits this district or that, are alike constant? Well, no matter how great the variation, no matter how much this voice falls below or exceeds the standard, judged from the human standpoint, attained by that, even we, with our duller perception, have no difficulty in recognising the species to which the owner of the voice belongs; in other words, the song is always specific, and this is the most noticeable, as it is the most remarkable, characteristic.

There is still, however, another quality to which I would draw attention—that of loudness. The sounds produced are on the whole alike penetrative, and the individuals of any given district, even though the climate by affecting their vocal muscles may have modified the character of the song, are at no disadvantage in this respect; neither are the females on the same account the less likely to hear the undeveloped voice of the immature male.

We have then the following considerations: firstly, there is the widespread and remarkable phenomenon of imitation, from which we can infer that the diversity of song is not due to structural differences but must be sought in some innate capacity to play one tune in preference to another; secondly, not all the individuals of the same species play a similar tune—we find that there is in certain directions a noticeable variation which nevertheless does not seem to affect the question of success or failure in the attainment of reproduction; in the third place, in contrast with this variation, we can observe a striking uniformity in two important particulars, namely in the specific character and penetrative power of the song—qualities which we know are essential for the purposes of "recognition" and "warning"; and finally, from the general course of our investigation, we can infer that if a male had no certain means of advertising its position, the territory would not be brought into useful relation in its life. Have we here sufficient ground on which to construct a theory of origin; in other words, has the evolution of song been incidental to, and contributory to, the evolution of the territory?

We have all along spoken of the song and of the call-notes as if they were manifestations of separate emotional states having their respective and well-defined spheres of usefulness; and while, speaking generally, this is a true statement of the case, there is much evidence to show that the relationship between them is nevertheless very close. There are, for example, quite a number of cases in which a particular call-note is uttered with unusual energy during sexual emotion, and is attached to the song, of which it may be said to form a part; but a still closer connection can be traced in many simple melodies which are merely compositions of social and family calls repeated many times in succession, and even in some of the more complex productions there will be found indications of a similar construction. And since this is so, since moreover, in the seasonal vocal development of such a bird as the Yellow Bunting, we can observe the gradual elaboration from simple to complex—from the repetition of single notes to phrases and from phrases to the complete melody—we have every reason to suppose that it is along these lines that the evolution of the voice has proceeded.

In all probability there was a time when vocal expression was limited to primitive social and family cries which would be called into play with special force during times of excitement, more particularly during the sexual season which is the period of maximum emotional excitement. But the excitement would express itself in all the congenital modes of behaviour peculiar to the season, and thus the repetition of these cries would become associated with combat, with extravagant feats of flight, and with other forms of motor response. Now the more emotional individuals would be the more pugnacious, and all the more likely therefore to secure territory and so to procreate their kind; and, being of an excitable disposition, they would at the same time be the more vociferous. Hence variations of the hereditary tendency to vocal expression, even though in themselves they were not of survival value, would be fostered and preserved, so long as they were not harmful, in virtue of their association with pugnacity. But if, instead of being neutral, they helped to further the biological end of combat, the relationship between the voice and pugnacity would be of a mutually beneficial kind; and those individuals in which variation in both directions happened to coincide, would have a better chance of success in the attainment of reproduction.

A territorial system, closely corresponding to that which we have discussed, forms part of the life behaviour of certain mammals, and of its existence much lower in the scale of life evidence is not wanting; from which we can infer that it is not of recent origin, but that the conditions in the external environment demanded such a system at a remote period of avian development. Now even in its incipient stages the system must have involved a separation of the sexes, and howsoever slight the degree of separation may have been in comparison with that which can be observed to-day, inasmuch as the power of locomotion was then less highly developed, mating could only have proceeded satisfactorily providing that males fit to breed had some adequate means of disclosing their positions. Thus there is reason to think that from the very commencement of the process variations of emotional disposition expressed through the voice would have been of survival value.

But expressed in what direction, in loudness and persistency of utterance, these are the qualities which, I imagine, would have been more likely to have facilitated the search of the female? Yet if she were uncertain as to the owner of the voice, neither loudness nor persistent repetition would avail much; and as species multiplied and the competition for the means of living became increasingly severe, so the necessity of a territory would have become intensified, and so, too, with the extension of range, would the separation of the sexes have been an ever-widening one; and as with their multiplication, irregularities and delays in mating, arising from the similarity of the calls, would have increased in frequency, so a distinctive call, which would have tended to minimise these risks, would have come to possess biological value.

Here we have a theory of origin, but origin of what? Of certain characteristics of song—nothing more; and therefore to suppose that it furnishes a complete explanation, which satisfies all the requirements of scientific logic, of so wonderful an intonation as that, for example, of the Marsh-Warbler, or that no other relationships, except that of the territory, enter into the total emotional complex, simplifying here or elaborating there to meet the exigencies of diverse circumstances—to suppose this would be foolish. That there are many relationships which even to-day are leading to modifications in important particulars, but which at the present time are beyond our cognisance, of this there can be no doubt.

There is one process by which song may have attained a fuller development, and which would account in some measure for the elaboration, inexplicable merely in terms of "recognition." It is this: the effect of the sexual call upon the female cannot well be neutral, it must be either pleasurable or the reverse—it must, that is to say, be accompanied by some suggestiveness, and by suggestion I mean the arousing of some emotion akin to that of the male; and if there are degrees of suggestiveness, which well there may be, some males will mate sooner than others and some will remain mateless—this is the theory of sexual selection. The question to be decided here is whether the biological emphasis is on loudness, or specific distinctness, or pitch, or modulation, or the manner in which the phrases are combined—that is, on some qualities in preference to others—or whether the emphasis is on the whole. We have already seen, and it is well known, that there is much variation in the voices of different individuals of the same species, and thus the first condition of the theory is fulfilled. Now the conditions which lead to variation are threefold—immaturity, seasonal sexual development, and isolation. Of the three, the variation in the case of the immature bird is the most instructive; the tone is not so pure, the combination of phrases is incomplete, and elaboration is imperfect, and yet, notwithstanding all these imperfections, we can observe that the bird pairs as readily as does the adult. But even if we lacked this demonstrative evidence, we should still be justified in assuming that such must be the case, for we know from experience in the preservation of game, where there is no surer way of reducing the stock than by leaving too high a percentage of old cocks, that for the young bird to be at any disadvantage in competition with the adult is detrimental, if not disastrous, to the species. So that while there is plenty of evidence of variation in those particular qualities which appeal to our æsthetic faculties, there is at the same time evidence which demonstrates that such variations exercise no influence on the course of mating; and inasmuch as it is difficult to conceive of any voice departing more from the normal type in these particular qualities than the immature does from the adult, if there be degrees of suggestive influence, we must seek it in some other direction. There remain the two other characteristics which we found to be constant under all circumstances, namely, loudness and specific distinctness; and if, in addition to serving the purpose of disclosing the positions of the males, they serve to evoke some emotion in the female, which helps to further the biological end of mating, so much the more reason is there for their survival.

There can be no question that this ingenious and attractive theory, if it were true in its special application to song, would immensely simplify interpretation, and moreover that preferential mating would contribute not a little to the success of the whole territorial system. No one can deny the strength of the argument: that the sexual instinct, like all other instincts, must require a stimulus of an appropriate kind; that the effect of the sexual call upon the female cannot be neutral; and hence the probability that stimulation varies too; no one, I say, can question the strength of this evidence, and, one might add, of the evidence derived from the analogy of the human voice. But when we have said this, we have said all; and our acceptance of the hypothesis, so far as song is concerned, must remain provisional so long as the evidence remains but secondary evidence.

CHAPTER V

THE RELATION OF THE TERRITORY TO THE SYSTEM OF REPRODUCTION

In the first two chapters I tried to show that the inherited nature of the male leads it to remain in a definite place at a definite season and to become intolerant of the approach of members of its own sex, and that a result is thus attained which the word "territory" in some measure describes. But the use of this word is nevertheless open to criticism, for it denotes a human end upon which the highest faculties have been brought to bear, and consequently we have to be on our guard lest our conception of the "territory" should tend to soar upwards into regions which require a level of mental development not attained by the bird. It is necessary to bear this in mind now we have come to consider the meaning of the territory, or rather the position that it occupies in the whole scheme of reproduction.

Relationship to a territory within the interrelated whole of a bird's life serves more than one purpose, and not always the same purpose in the case of every species. We have only to glance at the life-histories of divergent forms to see that the territory has been gradually adjusted to suit their respective needs—limited in size here, expanded there, to meet new conditions as they arose. Now some may think that the theory would be more likely to be true if the territory had but one purpose to fulfil, and that one the same for every species; and they may see nothing but weakness in the multiplication of ways in which I shall suggest it may be serviceable. But such an objection, if it were raised, would arise from a mistaken conception, a conception which, instead of starting with a relationship and working up to the "territory," sees in the "territory" something of the bird's own selection and thence works back to its origin. Holding the view that it is nothing but a term in a complex relationship which has gradually become interwoven in the history of the individual, I see no reason why the fact of its serving a double or a treble purpose should not be a stronger argument for its survival. I now propose to examine the various ways in which the territory may have been of use in furthering the life of the individual, and the circumstances in the inorganic world which have helped to determine its survival.

The purpose that it serves depends largely upon the conditions in the external environment—the climate, the supply of food, the supply of breeding-stations, and the presence of enemies. Hence its purpose varies with varying conditions of existence. But before we proceed to examine the particular ways in which it has been modified to suit the needs of particular classes of species, and the reason for such modifications, we must inquire whether there is not some way in which it has been serviceable alike to every species, or at least to a large majority of them.

Success in the attainment of reproduction depends upon the successful discharge of the sexual function; and the discharge of the sexual function depends primarily upon an individual of one sex coming into contact with one of the opposite sex at the appropriate season and when its appropriate organic condition arises. Now the power of locomotion is so highly developed in birds that it may seem unreasonable to suppose that males and females would have any difficulty in meeting when their inherited nature required that they should do so, still less reasonable to suggest that this power might even act as a hindrance to successful mating. Nevertheless, if we try to picture to ourselves the conditions which would obtain if the movements of both sexes were in no wise controlled, and mating were solely dependent upon fortuitous gatherings, we shall come, I fancy, to no other conclusion than that much loss of valuable time and needless waste of energy would often be incurred in the search, and that many an individual would fail to breed just because its wanderings took it into districts in which, at the time, there happened to be too many of this sex or too few of that. And as the power of locomotion increased and the distribution of the sexes became more and more irregular, so the opportunity would be afforded for the development of any variation which would have tended to facilitate the process of pairing, and by so doing have conferred upon the individuals possessing it, some slight advantage over their fellows.

What would have been the most likely direction for variation to have taken? Any restriction upon the freedom of movement of both sexes would only have added to the difficulties of mating; but if restriction had been imposed upon one sex, whilst the other had been left free to wander, some order would have been introduced into the process. That the territory serves to restrict the movements of the males and to distribute them uniformly throughout all suitable localities, there can be no question; and since the instinctive behaviour in relation to it is timed to appear at a very early stage in the seasonal sexual process, the males are in a position to receive mates before the impulse to mate begins to assert itself in the female.

We will take the Ruff as an example. According to Mr. Edmund Selous, pairing, in this species, is promiscuous—the Ruffs are polygamous, the Reeves polyandrous. Suppose, then, that upon this island of some few miles in circumference, whereon his investigations were made, the movements of neither Ruff nor Reeve were subject to control, that the birds wandered in all directions, and that the union of the sexes were fortuitous, would the result have been satisfactory? We must remember that the Reeve requires more than one Ruff to satisfy her sexual instinct; we must also bear in mind the possibility that the functioning of her instinct may be subject to some periodicity, and we ask whether, under these circumstances, accidental gatherings would meet all the requirements of the situation. Now, manifestly, she must be in a position to find males when her appropriate organic condition arises. But in the absence of any system in the distribution of the sexes, how could delay be avoided, or how could a uniform discharge of the sexual function be assured? There is, however, a system. In the first place, there are the assembly grounds to which the birds repair season after season; and then, on the assembly grounds, there are the territories, represented, as Mr. Selous tells us, by depressions where the grass by long use has been worn away, and each depression is owned by one particular Ruff. The assembly grounds have the effect of splitting up and scattering the birds, and the number of Ruffs at any one particular meeting place is limited by the territories; with the result that Ruffs fit to breed are evenly distributed and always to be found in certain definite places, and the Reeves know by experience where to find them.

The advantage of this territorial system is therefore apparent. Instead of this district being overcrowded and that one deserted; instead of there being too many of one sex here and too few of the other sex there; instead of a high percentage of individuals failing to procreate their kind, just because circumstances over which they have no control prevent their discovering one another at the appropriate time—each sex has its allotted part to play, each district has its allotted number of inhabitants, and the waste of energy and the loss of time incurred in the process of mating is reduced to a minimum.

Let us return again to the question of fortuitous mating, and consider the position of a male and female that have discovered one another by accident and have paired; what will be the subsequent course of their behaviour? We are assuming, of course, that a territory forms no part of their life-history. If the discharge of the sexual function takes place immediately and the ovaries of the female are in an advanced state of seasonal development, the construction of the nest will proceed without delay—and the nest will answer the same purpose as the territory in so far as it serves to restrict the movements of the birds and tends to make them remain in, or return to, its vicinity; but if not, there will be an interval during which both sexes will continue to wander as before, guided only by the scarcity or abundance of food. In the first case, there will be the attraction of the nest to prevent any untimely separation; in the second, there will be nothing in the external environment to induce them to remain in any particular spot. Now if we turn to any common species and observe the sequence of events in the life of different pairs, we shall find that pairing is seldom followed by an immediate attempt to build; that an interval of inactivity is the rule rather than the exception, and that this interval varies in different species, in different individuals, and in different seasons. Our imaginary male and female will therefore be faced with considerable difficulty; for with nothing in the external environment to attract them and with no restriction imposed upon the direction or extent of their flight, their union will continue to be, as it began by being, fortuitous. Next, let us consider their position were a disposition to establish a territory to form part of the inherited nature of the male. Each one will then be free to seek food when and where it wills and to associate with other individuals without the risk of permanent separation from its mate; and, no matter how long an interval may elapse between mating and nest-building, each one will be in a position to find the other when the appropriate moment for doing so arrives. Hence, while preserving freedom of movement for each individual, the territory will render their future, as a pair, secure.

No doubt the course of behaviour, as we observe it to-day in the lives of many species, is the outcome of, rather than the condition which has led to, the evolution of the territory. Thus, in many cases, we find that early mating is the rule rather than the exception; we find that the sexes frequently separate to seek their food, and fly away temporarily in different directions; and, under exceptional climatic conditions, we find that they even revert to their winter routine and form flocks; only, however, to return to their territories, as pairs, under more congenial conditions. Yellow Buntings, for example, pair comparatively early in the season—some in the latter part of February, others in March, and others again in April; and some build their nests in April, others in May. There is a gorse-covered common which I have in mind, a favourite breeding resort of this species. Between this common and the surrounding country, the birds constantly pass to and fro. If you watch a particular male you will observe that it sings for a while in its territory, that it then rises in the air and disappears from view, and finally that it returns to the tree, bush, or mound which constitutes its headquarters, where it again sings. Meanwhile the female, with which there is every reason to believe that this male has paired, behaves similarly; she, too, flies to the surrounding country and in time returns with equal certainty. Sometimes male and female accompany one another—that is, they leave simultaneously and likewise return; at other times, though they depart together, the male returns alone; or the male may disappear in one direction whilst the female does so in another—and, on the whole, there is a sameness in the direction of flight taken by the same pairs on different occasions. An interval of nearly two months may thus elapse between mating and nest-building, during which the sexes are not only often apart but often separated by a considerable distance.

What does this species gain by the individuals belonging to it mating so early in the season? If the appropriate condition which leads the females to seek males were to arise in each individual at a late date, the first stage in the process—mating—would not be completed before the second—the discharge of the sexual function—were due to begin. Thus, instead of having ample time, the females would have but a short period in which to discover males; and this in some cases might lead to delay, in others to failure, and in others again to needlessly severe competition, entailing physical exhaustion at a critical moment in their lives. Hence those females in which the appropriate organic condition developed early in the season would not only be more likely to find males, but would be in a position to rear more broods than those in which it developed late; and they would have a better chance of leaving offspring, which, in their turn, would reproduce the peculiarities of their parents. Moreover, within certain limitations, the more these successful females varied in the date of their development, the less severe would be the competition, and the more uniformly successful would the mating of all the individuals in a given district tend to become. But all of this renders an interval of sexual inactivity unavoidable; an interval which must constitute a danger unless there were something in the external environment to prevent the male and female from drifting apart. Inasmuch, then, as the occupation of a territory serves to remove all possibility of permanent separation, I suggest that its evolution has afforded the condition under which this beneficial procedure has developed—free to mate when they will, free to seek food where they will, free to pursue their normal routine of existence, and to meet all exigencies as they arise in their ordinary daily life—whilst free to do this, their future, as a pair, is nevertheless secure.

Thus far we have considered the territory in its relation to the discharge of the sexual function. In many of the lower forms of life, the success or the failure of reproduction, so far as the individual is concerned, may be said to end with the completion of the sexual act—the female has but to deposit her eggs in a suitable environment and then her work is done, because in due course and under normal conditions of temperature the young hatch out, and from the first are able to fend for themselves. And so, when we come to consider the question of reproduction in the higher forms of life, we are apt to focus attention too much upon the sexual function and too little upon the contributory factors, the failure of any one of which would mean failure of the whole. For a bird, success in the attainment of reproduction does not merely imply the successful discharge of the sexual function; much more is demanded; it must find somewhere to build its nest and to lay its eggs, it must shield its young from extremes of temperature and protect them from enemies, and it must be in a position to supply them with food at regular intervals. And, consequently, every situation is not equally favourable for rearing young; there must be a plentiful supply of food of the right kind in the immediate vicinity of the nest, and it must be in greatest abundance just at the moment when it is most urgently needed—that is to say, during the first few weeks after the birth of the young. Success, therefore, depends upon manifold relationships which centre in the station, and these relationships vary in intensity with the conditions of existence.

First, then, let us examine the problem from the point of view of the food-supply. There are many species whose success in rearing offspring is largely dependent upon the rapidity with which they can obtain food; and it makes but little difference which species we choose out of many—Finch, Bunting, Warbler, or Chat. I shall choose the Buntings, as their life-history in broad outline conforms to the general type, and, moreover, their behaviour is fresh in my mind. The young are born in a very helpless state; they are without covering —fragile organisms, ill-fitted, one would think, to withstand extremes of temperature, and wholly incapable of protecting themselves from enemies of any description. For the first three days after they are hatched the female spends much of her time in brooding them, and, when she is thus occupied, the male sometimes brings food to her, which she proceeds to distribute or swallows. But all the young cannot be fed, neither are they ready to be fed, at the same moment; and the parents have besides to find food for themselves, and the nest has to be cleaned—all of which necessitates the young being exposed to the elements at frequent intervals. Now it is impossible to observe the instinctive routine of the parents, when the young need attention, without being impressed with the conative aspect of their behaviour. Why, we ask, are the movements of the female so brisk; why does she seek food and clean the nest so hurriedly; why, if her instinctive routine is interrupted, do her actions and her attitude betray such bewilderment? I take it that the only answer we can give to these questions is that the part of her inherited nature which predominates just at this particular time is to brood. But why is brooding of such importance? Partly to maintain the young at the proper temperature, and thereby to induce sleep—and sleep for offspring newly hatched is as important as food—and partly to protect them from the risk of exposure to extremes of temperature. This latter danger is no imaginary one. Examine a young bird that has recently left the egg; observe its nakedness; and consider what it has to withstand—a temperature that may rise to 70° F. or may fall to 40° F., the tropical rain of a thunderstorm or the persistent drizzle of many hours' duration, the scorching effect of a summer sun or the chilling effect of a cold north-easterly wind, and, constantly, the sudden change of temperature each time that the parent leaves the nest. One marvels that it ever does survive; one marvels at the evolution of a constitution sufficiently elastic to withstand such changes. But, however much the constitution may give us cause to wonder, it is clear that much depends upon the parents. A slight inefficiency of the instinctive response which the presence of the young evokes, a little slowness in searching for food or sluggishness in returning to the nest, might lead to exposure and prove fatal. And, however much is demanded of the parents, it is clear that much also depends upon the relationships in the external environment; for no matter how sensitive or how well attuned the instinctive response of the parent may be, it will avail but little in the presence of unfavourable conditions in the environment.

Everything turns upon the question of the effect of exposure. And in order to ascertain how far extremes of temperature are injurious, I removed the nests of various species containing newly hatched young, and, placing them in surroundings that afforded the customary amount of protection from the elements, I made a note of the temperature and the atmospheric conditions and then observed the condition of the young at frequent intervals. Details of these experiments will be found at the end of the chapter.

The experiments with the Blackbirds and the Whitethroats gave the most interesting results. Both broods of each species were respectively of much the same age, yet one brood of Blackbirds survived for five, and the other only for two and a half hours, and one brood of Whitethroats lived for twelve hours whilst the other succumbed in a little over an hour. This difference is rather remarkable; and it seems clear that the power of resistance of the young diminishes rapidly when the temperature falls below 52° F. It must be borne in mind, however, that the conditions under which the experiments were made were, on the whole, favourable—the weather was dry, the temperature was not unusually low, nor was the wind exceptionally strong or cold; and even in those cases in which the young succumbed so rapidly, the atmospheric conditions could by no means be regarded as abnormal.

What, then, would happen in an unusually wet or cold breeding season? For how long would the young then survive? In the spring and early summer of the year 1916, I was fortunate in observing the effect of exposure under natural but inclement conditions. I happened to be watching the Yellow Buntings on Hartlebury Common—200 acres of Upper Soft Red Sandstone, profusely overgrown with cross-leaved heath (Erica tetralix), ling (Calluna vulgaris), and furze (Ulex)—in one corner of which eight males had established adjoining territories covering some fifteen acres of ground. The males obtained mates towards the end of March or at the beginning of April; nests were built in the middle of May, and the successful pairs hatched out their young in June. On the 10th June the weather became exceptionally cold, and during the next ten days the temperature fell at times to 40° F. during the daytime. Slight frosts were registered at night in the district, and the young bracken, which covered the Common in places, had the appearance of having been scorched and eventually withered away. At the coldest period of this cold spell the young were hatched in two of the nests—in the first one on the 10th June, and in the second a day or so later; and on the morning of the 10th June, having found a suitable position near the first nest, I began to watch the movements of the parents, with the intention of keeping some record of their behaviour each day so long as the young needed attention. An hour passed without their appearing, and on examining the young I found that they were cold, feeble, and unresponsive, but the female presently arrived and went to the nest. Later in the day the young were lively and responded freely when the nest was approached, but nevertheless I was impressed with the length of time during which the parents were absent; for, judging by the experience of previous experiment, there seemed to be every likelihood of their losing their offspring in such abnormally cold weather, unless they brooded them more persistently. On the 11th June at 5.50 A.M. neither parent was to be seen and the young could scarcely be made to respond; but shortly afterwards both male and female appeared, and, after remaining a few minutes, again disappeared without even approaching the nest. At 6.45 A.M. no attempt had been made to brood and the young were then so feeble that they were scarcely able to open their mouths, and at 6 P.M. one was still alive but the remaining three were dead. Yet the parents returned and the female went to the nest; and, from a distance of a few feet, I watched her brooding the living and the dead. At 5.45 A.M. the following day the remaining young bird had succumbed, the temperature then being 49° F.

At the second nest, I was unable to watch the behaviour of the parents so closely. On the 15th June the nest contained three young from three to four days old, and during the morning of that and the succeeding day nothing unusual occurred, with the exception that the period of exposure seemed, as in the former case, to be too long. On the 17th June at 3.10 A.M. the young had collapsed and were stiff, but the parents were in their territory and anxious apparently to attend to their brood. At 9.15 A.M. only two of the young were left in the nest, and though I searched amongst the undergrowth and in the gorse bush in which the nest was placed, no trace of the third bird was to be found. Of the two remaining young, one was alive and responsive but the other was dead, and though the female attended assiduously to the sole surviving offspring, yet it too had succumbed by the following morning.

In a third territory, there was a nest containing four eggs. These eggs were due to hatch at much the same time as those in the two nests just referred to, but they failed to do so, and an examination showed that they contained well developed but dead chicks.

To what can the death of the young and of the chicks in the eggs be attributed? Not to any failure in the instinctive response of the females, for they fed their young, they brooded them, they even brooded the dead as well as the living, and probably did all that racial preparation had fitted them to do. Yet the fact that the young in the second nest were lifeless and exposed at 3 A.M. seems to betoken absence on the part of the parents during the night, and may be interpreted as a failure of the parental instinctive response. Let us return for a moment to the experiments. These showed, it will be remembered, that a rise or fall in the temperature of but a few degrees was sufficient to make an astonishing difference in the length of time that the young were able to survive without their parents; that when the temperature reached 58° F. the bodies of the young retained their warmth, and that under such conditions even a night's exposure had little, if any, effect; so that even supposing that the parents were absent during the night, the death of the young cannot be said to have been due to a failure of the parental instinct, because under normal conditions—and under such has their instinctive routine been evolved—their absence would not have prejudiced the existence of the offspring. I attribute the collapse of the young solely to the exceptional cold that prevailed at just the most critical time, and I base this conclusion partly on the experience gained from experiment, but mainly on their condition observed at different intervals; for during exposure they collapsed rapidly, their flesh became cold and their movements sluggish, their response grew weak, and gradually they became more and more feeble until they could scarcely close their bills after the mandibles had been forced asunder. Yet, even after having reached so acute a stage of collapse, the warmth from the body of the brooding bird was sufficient to restore them temporarily; once more they would become lively and responsive, only, however, to revert to the previous condition soon after the parent had again abandoned them. Doubtless their power of resistance grew less and less during each successive period of exposure.

If the nestling Bunting is to be freed from the risk of exposure, it is evident that there must be, in the vicinity of the nest, an adequate supply of food upon which the parents can draw liberally. Hence those pairs that exercise dominion over the few acres surrounding the nest, and are thus able to obtain food rapidly, will stand a better chance of rearing their offspring than others which have no certain supply to draw upon—and this, I believe, is one of the biological ends for which the territory has been evolved. But it must not be supposed that each pair finds, or even attempts to find, the whole of the food within its territory, or that it is necessary for the theory that it should do so; all that is required is that such overcrowding as might lead to prolonged absence on the part of the parents and inordinate exposure of the young shall be avoided. So that the problem has to be considered not merely from the point of view of the individual, but from the larger point of view of all the pairs inhabiting a given area.

Now there were eight pairs of Yellow Buntings occupying the one corner of Hartlebury Common, and their territories in the aggregate covered some fifteen acres. The birds obtained part of their food-supply amongst the gorse and in some young scattered oak-trees, and part in an adjoining coppice and on the surrounding arable land. But they were not the sole occupants of this corner of the Common; other insectivorous species had territories there also—amongst which were Whitethroats, Grasshopper-Warblers, Willow-Warblers, Whinchats, Stonechats, Meadow-Pipits, Tree-Pipits, and Skylarks. Suppose then that there had been sixteen pairs of Yellow Buntings instead of eight; that there had been other pairs, which assuredly there were, inhabiting the locality; that they had also resorted, which assuredly they did, to the coppice and arable ground for the purpose of securing food; and that their numbers had also been increased in a similar ratio—would a supply of food for all have been forthcoming with the necessary regularity and promptitude? Well, the parents might have had to travel a little farther; but even if they had been compelled to do so, their absence would only have been prolonged by so many minutes the more, and under normal conditions what harmful result to the offspring could possibly have followed? The question for us, however, is not what might have occurred under normal conditions, but whether the life behaviour is so adjusted as to meet the exigencies of diverse, and in this case of abnormal, circumstances. Now the capacity of the young to resist exposure diminishes very rapidly when the temperature falls below the normal—the danger zone seems to be reached at approximately 52° F., and the length of time during which they survive then becomes astonishingly short—and moreover the fall in the temperature would tend to decrease the supply of insect life upon which they depend, so that if the size of the territories had been reduced by one half, and the parents in consequence had been compelled to seek their food at a greater distance, can it be doubted that the cumulative effect of even a few minutes of additional exposure would have been detrimental, if not disastrous, to the offspring?

We speak, however, of the parents extending their journeys a little farther in this direction or a little farther in that, as though they could do so with impunity except in so far as it affected themselves, or their offspring, or the other Yellow Buntings inhabiting that particular area. But, most certainly, any extension would have meant so much encroachment upon the available means of support of other members of the species inhabiting adjoining areas, whose young in turn would have been liable to have been affected; and, with even greater certainty, the Whitethroats, the Stonechats, the Tree-Pipits, and the Willow-Warblers that had also established themselves in that one corner of the Common would have been hard pressed to find sufficient food with sufficient rapidity.

Let me give another illustration of a somewhat different kind. Lapwings, as we saw in the previous chapters, establish territories and guard them from intrusion with scrupulous care. The young are able to leave the nest soon after they are hatched, and consequently the parents are not necessarily obliged to bring food to them—they can, if they so choose, lead them to the food. Whether each pair limits its search for food to its territory, I do not know. But even supposing that all ownership of territory were to lapse directly the young were hatched, that the boundaries were to cease to exist, and that the birds were free to wander at will without fear of molestation, the end for which the territory had been evolved would none the less have been obtained; for inasmuch as the parents are accompanied by their young, it matters not in what part of the meadow they seek their food; all that matters is that the number of families shall not exceed the available supply of food. So far, then, as the Lapwing is concerned, the territory fulfils its purpose when once it limits the number of males, since, by doing so, it limits the number of families and prevents undue pressure upon the means of support.

Nevertheless, there are many birds that seem to rely entirely upon the territory to supply them with all that is necessary. Each Warbler seeks its food within the precincts of its own particular domain, and, except in occasional instances, neither resorts to neutral ground nor makes excursions into the locality immediately surrounding the territory, as does the Bunting. Probably it would be disastrous if it attempted to do so, for since its young at birth are so delicate and so susceptible to changes of temperature, it cannot afford to be absent from them for long. Of the two experiments made with young Whitethroats, one was made under favourable and the other under unfavourable conditions. In this latter case the temperature was 50° F., and the young, it may be remembered, only survived for a little over one hour. Now exposure at that temperature is evidently dangerous, but it would be still more dangerous if the weather were wet instead of dry, and the temperature 46° F. instead of 50° F.; and it is, I imagine, on this account that the impulse to brood is so strongly implanted in the female. No sooner, it seems, does she depart than she returns with a small quantity of food which she hurriedly distributes and immediately settles down to brood; and if forcibly prevented from returning, her attitude betrays symptoms of what, humanly speaking, we should term great distress. If, then, the conditions in the external environment were such as would make it difficult for the female to obtain food rapidly, what advantage would she derive from so strongly developed an impulse? Might it not be a disadvantage? Might it not mean that she would abandon the search too readily and be content to return with an insufficient supply, and might not that be as injurious to the young as prolonged exposure? Manifestly the impulse to brood could only have developed strength in so far as it fitted in with all the other factors that make for survival; and the principal factor in the external environment seems to be the territory. How could the young have been freed from the risk of exposure if the impulse to brood had not been so strongly implanted in the parent? How could the impulse to brood have been free to develop if a supply of food had not been first insured? How could the supply of food have been insured if numbers of the same species had been allowed to breed in close proximity?

From the foregoing facts it is clear that the young of many species are at birth susceptible to cold and unable to withstand prolonged exposure. The parents must therefore be in a position to obtain food rapidly, and consequently it is important that there should be an ample supply in the vicinity of the nest. This end the territory certainly serves to promote; it roughly insures that the bird population of a given area is in proportion to the available means of subsistence, and it thus reduces the risk of prolonged exposure to which the young are always liable.

This leads on to a consideration of those cases in which the question of securing food is subordinate to the question of securing a station suitable for reproduction.

I take the Guillemot as an example. In principle its behaviour is similar to that of the Bunting; the male repairs to a definite place, isolates itself, and becomes pugnacious. But the Guillemot is generally surrounded by other Guillemots, and the birds are often so densely packed along the ledges that there is scarcely standing room, so it seems, for all of them. Nevertheless the isolation of the individual is, in a sense, just as complete as that of the individual Bunting, for each one is just as vigilant in resisting intrusion upon its few square feet as the Bunting is in guarding its many square yards, so that the evidence seems to show that that part of the inherited nature which is the basis of the territory is much the same in both species. What we have then to consider is, What is the biological value to the Guillemot of an inherited nature which, for the Bunting, has utility in relation to the supply of food for the young? Up to a point, the act of securing a territory has like value for each respective species, whether the area occupied be large or small—that is to say, it enables the one sex to discover the other with reasonable promptitude.