# Territory in Bird Life/Chapter 7

CHAPTER VII
THE RELATION OF THE TERRITORY TO MIGRATION
Coincident in time with the growth of appropriate conditions in the environment, organic changes take place rendering certain instincts susceptible to stimulation; and the stimulus being applied, the Warbler leaves the country wherein it had passed the winter and finds its way back, with apparently little difficulty, to the district in which it was reared or had previously reared offspring. What is the nature of these changes and of the impulse which is first brought into functional activity; whence comes the stimulus; and what directs the bird on its journey—these are all different aspects of one great problem, the problem of migration. I do not propose to discuss all these various aspects, for indeed I have no suggestions to offer which are in the least likely to be helpful, but I seek rather to ascertain whether the phenomena which we have explored bear any relation to the problem as a whole; whether, that is to say, the competition for territory and all that appertains to it can have ​supplied the conditions under which, in the process of time, this complex and definite mode of behaviour has evolved.
We are sometimes told that we must seek the origin of migration in the physical changes that have occurred in the ancient history of the earth—in glacial conditions which gradually forced birds to the south, or in the "stability of the water and mobility of the land" which brought about a gradual separation of the feeding area from the breeding area—and which continued for a sufficient length of time to lead to the formation of an instinct, and that the instinct persists because it is serviceable in promoting the welfare of the race. But when we consider the lapse of time, and the changes that must have occurred in the character of the bird population—the appearance of new forms and the disappearance of the old, the ebb and flow of a given species in a given area—and bear in mind that, notwithstanding this, the migratory instinct, if not stronger, is assuredly no less strong, and the volume of migration, if not greater, is assuredly no less; in short, that the whole phenomenon is progressive rather than retrogressive, we shall find the view that the instinct owes its origin to conditions which no longer exist, receives but little encouragement.
I doubt not that, throughout the ages, geological changes have been an important factor in directing or limiting the scope of migration, and moreover are so still; just as ​climatic changes and the relative abundance or scarcity of enemies have influenced the course of its evolution. These are all contributory factors operating in the external environment. But there are, besides, internal factors which form part of the inherited constitution of the bird, and, being passed on from generation to generation, afford the conditions under which migration is constantly being renewed. It is, I believe, in this field of organic change and relationship that the conditions of origin must be sought.
Just as the moth in passing from the rudimentary to the perfect condition runs through a series of changes, each one of which is marked by a typical behaviour response adjusted to meet some particular circumstance in the external environment, so the annual history of a bird displays an ordered routine, each phase of which can be observed to correspond with one of the successive changes in the environment. In almost every direction, we find that this routine is characterised, in broad outline, by great uniformity; so much so that, providing we know the history of one species, we can forecast with no small degree of certainty the general course of behaviour of other members of the family. But only the general course. There is endless variation in just the particular way in which the behaviour is adapted to meet the needs of particular species—the major details may be said to be specific, the minor details varietal.
​Now it is that part of the behaviour routine which has reference to the relationship between one bird and another upon which, for the time being. I wish to dwell; for the interest that A displays in B is by no means always the same—it changes according to the season, and this change can be observed to be uniform throughout a wide range of species.
In winter, in whatsoever direction we turn, we observe not only that different individuals but that different species also collect together in flocks. And since food at that season is not always easy to obtain, and, moreover, is only to be found in certain situations, which are limited both in number and extent, it would seem that such assemblages are in the main determined by accident. No doubt the abundance or the scarcity of food does determine the movements of birds, and hence to that extent may be held to account for the flocks. But we shall but deceive ourselves if we think that it is the sole or even the principal reason, or that the situation is in no wise affected by internal factors. The behaviour of the individual in relation to the flock bears ample testimony to the presence of a gregarious impulse which derives satisfaction from the fact of close association.
As an illustration, let us take a bird whose movements are easily watched, and in whose hereditary constitution the impulse to which I allude seems to be strongly implanted—the Curlew. When the breeding season is over. Curlew leave the mountain and the moor and ​return to the coast or tidal estuaries for the remainder of the year. Here, at low water, they find an abundant supply of food—crustaceans amongst the sea-weed upon the rocks, and lobworms (Arenicola piscatorum) in the mud as the tide advances or recedes. But when the tide is full, they retire to those parts of the shore that remain uncovered—to isolated rocks, or to sanddunes, or it may even be to pasture-land in the neighbourhood. During this period of repose large numbers of individuals gather together on a comparatively small space of ground. They are not constrained to do so by any shortage of accommodation, nor by any question relative to food, nor, for the matter of that, by any circumstance in the external environment; they are brought together solely, this at least is the impression that one gains, by some inherited impulse working towards that end. And their subsequent course of behaviour tends to confirm that impression. For if we watch the gathering together of the different units of which the flock is composed, and study more particularly the emotional manifestation which accompanies their arrival and departure, we shall find that the coming of a companion arouses some emotion which is expressed by a vocal outburst that sweeps through the flock.
Now each call, and the Curlew has a great variety, is not only peculiar, generally speaking, to certain occasions, but is accompanied by a specific type of behaviour, whence we can infer in broad outline the type of emotion which is ​aroused. Thus we come to recognise fear, anger, or sexual emotion, by just the particular sound which is emitted. But even if we are going too far in referring particular calls to particular emotions, we can, without a doubt, divide them into two broad categories—those which are pleasurable and those which are the reverse. And we need have no hesitation in placing the particular call to which I allude in the first of these two categories, not only on account of the nature of the sound produced, but because the activities which are aroused are not such as normally accompany irritation. This is well seen if the behaviour of different individuals be closely observed. After resting on one leg for some time, first one and then another is seized with cramp, and running a few yards in an ungainly way, bumps up against its companions as if it had not full control over its movements. Its behaviour produces irritation which is expressed by a vocal outburst, and followed by actions the meaning of which is clear. Moreover, the call is taken up by other individuals and sweeps over part of the flock as does the greeting. But the nature of the cry is entirely different from that which greets the arrival of a companion—humanly speaking it is a passionate and impatient utterance, the height of displeasure. The arrival, then, acts as a stimulus to something in the inherited constitution which is expressed in, and presumably is satisfied by, this vocal outburst; and, since the bird that arrives joins also in the ​chorus, there is reason to think that the impulse which determines its movements is similar to that which is temporarily aroused in the flock.
Apart, however, from the evidence derived from the affective aspect of the operation of the instinct, the general course of behaviour lends support to the view that the assemblies are determined by internal factors, and are not merely the outcome of circumstances in the external environment. Observe, for example, the manner in which the flock is built up. Single individuals are content to rest alone so long as no assembly is in sight, but they are drawn towards their companions directly the opportunity arises, just as surely as the smaller aggregation is drawn towards the flock; and so, as the flock increases, it gradually absorbs all the lesser flocks and smaller parties, for the greater the flock the greater the attraction seems to be; and different individuals appear to gain some satisfaction from being in close bodily contact with one another.
When the Curlew flies to that part of the mud-flat which is first exposed by the receding tide, and there associates with others, it does not then do so because it has any interest in its fellows, nor because they serve as an attraction, but because it is constrained by hunger—in other words, the association is determined by accident. But when, during periods of repose, it sees a flock, flies to it, and takes up a position in the midst of it, it does so not because suitable accommodation is lacking—not therefore because ​of external constraint—but because it derives some pleasure from satisfying something in its organic complex. We speak of this behaviour and of the emotion which characterises it as the gregarious instinct: by which we mean that the inherited nature of the Curlew, as a tribe, is so constituted that, given the appropriate internal conditions and adequate external stimulation, every individual will respond in a similar manner—that is, the behaviour is primarily determined by racial preparation. This is what we mean by the gregarious instinct biologically considered. We may resolve our own experience in relation to the crowd into its simplest constituents, project our own primitive feelings into the Curlew, and say that the bird feels uneasiness in isolation and satisfaction in being one of the flock. But in truth we know nothing, save by analogy, of the correlated psychical state. All the knowledge we possess is derived from a study of the objective aspect of the behaviour, which in simple terms may be expressed thus: the individual is drawn towards its companions; there is a relation between the size of the flock and the strength of the attraction; and all Curlew behave similarly under similar circumstances.
This instinct controls the movements of many birds from early autumn to the commencement of the breeding season. And so powerful is the control that the individual is suppressed and its activities subordinated to the welfare of the community as a whole. Flocks ​of Waders roam about the tidal estuaries in search of food, and different kinds of Gulls assemble there and preen their feathers or sleep; Warblers alter their mode of life, and in the osier bed, or amongst the elders, seek their food together in peace; Finches, Buntings, Pipits, and Wagtails, though food is everywhere abundant, gather themselves together respectively into bands which, as winter approaches, grow into flocks and even into composite flocks; and as the Warblers leave for the south, so their places are filled by flocks of Thrushes and Finches from the north. In whatever direction we turn, when the days begin to shorten, it is the community, not the individual, that thrusts itself upon our attention; and throughout the winter continues to be the outstanding feature of bird life.
With the approach of the breeding season we witness that remarkable change which I have endeavoured to make clear in the previous chapters—the disintegration of the flock and the reinstatement of the individual. Instead of continuing with the flock, the individual now goes forth to seek the appropriate breeding ground; and having arrived there, is not only content to remain in isolation, but so behaves that isolation is insured. Intolerant of the approach of a stranger, intolerant even of the approach of the very members of the community whose companionship was previously welcomed, it not only fights to maintain the position it has selected, but fights indeed for the possession of ​ground already occupied, and, until reproduction is completed, asserts its individuality and exercises dominion over its territory. What, then, is the prospective value, biologically considered, of the changing interest that A displays in B, and to what will such changes lead? These are the questions to which we will now direct inquiry.
The annual life-history of a bird is in broad outline conditioned by two powerful and at first sight opposing impulses—the one to live in society, the other to live solitary. But, manifestly, a bird cannot be governed by opposing impulses. It has but one character, within which, according to the season and the circumstances, different impulses predominate. But these impulses, no matter how different they may appear to be, have their respective parts to play in furthering the life of the individual. Hence they cannot oppose, though they may conflict, if the resultant behaviour contributes towards survival.
The majority of birds live to-day in constant danger from predatory species, and that this danger was still greater in bygone ages there can be but little doubt. A curious mode of behaviour of the Curlew, Whimbrel, and Godwit demonstrates this, for it must be the outcome of the necessity for constant watchfulness. Whilst resting with its head turned back and its beak buried in the feathers of the mantle, the bird constantly moves the axis of its body, so that an observer, if placed in a direct line ​ behind it, sees at one moment the right eye and at another the left. No movement of the feet or of the legs is perceptible, and the shifting of the body continues whether the eyes are open or closed. This body movement enables the bird to survey a much larger area of ground than it would otherwise be capable of doing, and thus adds to its security. As far as my experience goes, the movement is less evident amongst the members of a flock than when an individual is resting, alone, or even with a few companions, which may be due to the fact that since some members are always awake and watchful, a bird of prey would have more difficulty in approaching a flock unawares than it would have in approaching a single individual. With the greatest ease a Sparrow-Hawk can pick up a Thrush as it feeds on the meadow by itself, but if it attempts to seize one of a flock, the chances are that its approach is signalled and that its prey escapes. And not only do the different members give warning one to another of the approach of danger, but they also combine to harass or even to drive away an enemy. So that there can be no doubt that the gregarious instinct is service- able in promoting the welfare of the race, and has, as its end, the preservation of the individual in order that it may take its share at the appropriate time in procreating its kind.
In winter, then, the individual loses its individuality and is subordinated to the welfare of the community, whilst in spring it regains ​its individuality, and all its inherited instincts which then come into operation lead to its isolation from the flock. The impulse to seek isolation is dependent upon internal organic conditions which are peculiar to a certain season; whereas the gregarious impulse depends upon internal organic conditions which inhere at all times, though its functioning is inhibited by the functioning of the former impulse. The evidence which leads to this conclusion is to be found in the fact that a male often deserts its territory temporarily and joins the flock, where it remains at peace with its companions—an aspect of behaviour which we have discussed on various occasions. The former impulse becomes dominant in the spring owing to its innately superior strength; the latter becomes dominant in the autumn because the organic condition which determines the functioning of the former then subsides. The impulse to seek the appropriate breeding ground and to dwell there would seem to be the strongest of all the impulses save one—the sexual. When, however. I speak of the sexual. I refer to the actual discharge of the sexual function, which is the consummation of the whole process. But the territory and all that appertains to it is part of that process—the search for the breeding ground, the dwelling there, and the intolerance of intrusion are but different stages, each one of which must have an impulse peculiar to it; and since the completion of the sexual act can only be ​successfully accomplished providing that success is attained at every stage, the probability is that, of the impulses concerned, one is neither more powerful nor less powerful than another. So that we have two impulses operating at different seasons and guiding the behaviour into widely divergent channels. But though the proximate end to which the behaviour is directed is apparently different, there are not two biological ends in view, but one—the attainment of reproduction; and the changes that we witness are not contrary but complementary, and their prospective value lies in the circumstance that they contribute towards the preservation of the race.
If, then, every male is driven by inherited impulse to seek the appropriate breeding ground each recurring season; if, having arrived there, it is driven to seek a position of its own; if, in order to secure isolation it is obliged to attack other males or to ward off the attacks of intruders; if, in short, success can only be attained providing that the inherited nature is so adjusted that the bird can accomplish all that is here demanded—what will be the general result? That the individual will rear its offspring in safety and that they will inherit the peculiarities of their parents, enabling them, in their turn, to procreate their kind; all this will certainly follow. We are not concerned, however, at the moment, with the direct effect upon the individual, but with the consequences that will accrue to the species as a whole.
​Now certain facts are presented to observation which enable us not only to understand the nature of the change that is wrought in the history of the species, but to foreshadow, with no small degree of certainty, the extent of that change. I suppose that it has come within the experience of most of us to observe, at one time or another, the ebb and flow of a given species in a given district. Some favourite haunt is deserted for a year, or for a term of years, and is then revisited; or, if it is always occupied, the number of inhabitants fluctuates—aplenty of pairs in this season, only a few in that. Many intricate relationships, both external and internal, contribute towards this state of affairs. Fluctuation in a downward direction, or temporary extinction, is brought about by changes in the physical world, by changes in the available supply of food, by the increase of enemies, or by adverse climatic conditions; whilst fluctuation in an upward direction, though due indirectly to a combination of circumstances in the external world favourable to the survival of large numbers of individuals, is directly determined by the impulse to seek isolation. As individuals of different species establish themselves, and form kingdoms and lesser kingdoms, we can watch the gradual quickening into life of moorland and forest and we can observe the manner in which it all comes to pass. Males that for weeks or months have lived in society, drifting from locality to locality according to the abundance of food or its ​scarcity, now set forth alone and settle first here and then there in search of isolation. Lapwings settle in the water meadows, and, finding themselves forestalled, pass on in search of other ground; Blackbirds arrive in a coppice or in a hedgerow and, meeting with opposition, disappear; and the Curlew, wandering with no fixed abode but apparently with a fixity of purpose, searches out the moorland where it can find the particular environmental conditions to which its inherited nature will respond. In fact, wherever we choose to look, we can observe in a general way the gradual appropriation of breeding ground; and if we fix our attention upon particular males, we can watch the method by which success or failure is achieved.
On more than one occasion I have watched the efforts of Reed-Buntings to appropriate territories in a marsh that was already inhabited. Sometimes their efforts met with success, at other times with failure. In the former case, the males, whose ground was intruded upon, were severally forced to yield part of their holding and were thus left in possession of a smaller area. The success of the intruder seemed to depend upon persistent determination, rather than upon superior skill in battle. Recently I had an opportunity of observing the intrusion of a male Willow-Warbler upon ground already occupied. By persistent effort it succeeded in appropriating one half of the territory of its rival. The intruder occupied some trees on the ​outskirts of the territory it was invading, and used them as a base from which it made repeated efforts to enter the ground of its rival. These efforts were time after time frustrated. No sooner did it leave its base than it was seen and intercepted, or else attacked; and no matter from which direction it attempted to effect an entrance, its efforts, for a time, were all to no purpose. The fighting was of a determined character, and after each attack the owner of the territory showed signs of great excitement, and, sitting upright upon a branch, spread and waved its wings, which is the specific emotional manifestation during the period of sexual activity. Eventually the intruding male succeeded by persistent effort in appropriating part of the occupied ground.
Thus we can actually witness the efforts of the individual to isolate itself from members of its own kind, and can observe the immediate consequences that follow from success or from failure. And from these consequences we can infer that, within a certain range but in accordance with the relative abundance of the species that dwell in it, every corner of the available breeding ground will be explored and every situation that evokes the appropriate response will be occupied. Moreover, since the annual dispersion is not merely a repetition in this season of that which occurred in a previous one, a progressive increase in the area occupied will follow. Yet, if the majority of species desert their breeding ground so soon as reproduction is ​ended, how can this be? An answer to the question will be found in the fact that a bird has an innate capacity to return to the neighbourhood of its birthplace, or to the place wherein it had previously reared offspring—which means that the results of prior process persist as the basis and starting-point of subsequent process.
Bearing then in mind that the seeming peace in bird life around us in the spring is but the expression of transitory adjustments in the distribution of individuals and of species; bearing in mind how widespread is the search for isolation each recurring season, how frequently the search leads to competition and competition to failure, and how failure implies a renewal of the search; bearing in mind that situations, which appear to be eminently suitable for breeding purposes, are passed by year after year and remain unoccupied, just because, for reasons which have yet to be ascertained, the environment fails to supply some condition which is essential if the inherited nature of the bird is to respond—can there be any doubt that the general result of the functioning of the disposition will be expansion; or, since no limit is placed upon it from within but only from without—that is, by unfavourable circumstances in the external world, that the expansion will not merely be in one direction but in every direction?
If now, when reproduction is ended, all the impulses relating to it die away, and the ​gregarious instinct again predominates, what are the consequences to which this change will lead? Just as the consequences which flow ixom the functioning of the former impulse are accessible to observation, so likewise can we observe the change that is wrought by the latter impulse. The process is a gradual one. Less and less attention is paid by the individual to intruders, more and more is it disposed to pass beyond its accustomed limits. Little by little, accompanied by its young or without them, as the case may be, the bird deserts its territory and wanders out into the wilderness. Here it associates with others, and finds in them a new interest and. I doubt not, a new enjoyment. All this we can observe as it takes place. But just as there is an innate capacity to seek, in the spring, the place where the pleasures of breeding had formerly been enjoyed, so we are bound to infer the existence in the adult of an innate capacity to revisit the former area of association; and this capacity will strengthen and confirm the gregarious instinct and set the direction of the general course of movement.
We have seen, then, that the interest displayed by one bird in another changes with the seasons; we have seen that it is so modified as to be in useful relation to different environmental circumstances; as far as possible we have traced out the consequences, and have reached the conclusion that the change of behaviour must, on the one hand, lead to expansion, and on the other, to contraction; ​and we have seen that this conclusion is in accord with the facts of observation—that is the general result of our inquiry into the functioning of the two powerful impulses, the impulse associated with the disposition to secure a territory and the gregarious impulse.
The phenomenon of migration embraces a number of separate problems, each one of which presents features of great interest and of still greater difficulty. On some of these problems I do not intend to touch; I seek only to ascertain whether the impulses that are concerned in the securing of a territory, and in the search for society, bear any relation to the problem as a whole. I hold that the origin of migration is not to be found merely in conditions peculiar to a remote past, but that the conditions inhere in the organic complex of the bird, and are thus handed down from generation to generation. Starting with this assumption I examined the behaviour which normally accompanies the seasonal life-history of the individual, and found, in that behaviour, manifestations of cyclical change leading to definite biological consequences. I now propose to inquire whether those consequences are such as might, in the course of time, give rise to the seasonal change of abode.
We are apt to think of migration in terms of the Warbler that enlivens our hedgerows in the spring after travelling hundreds of miles from the south, or of the Redwing that comes from ​the far north and seeks its food during the winter on the meadows, or perhaps of the American Golden Plover that each year covers a vast expanse of ocean in its journey from its breeding ground. The length of the distance strikes the imagination and constrains us to focus attention upon the extremes.
But migration is of much wider significance than is here represented. I sit beside the River Severn in April and watch Swallows. TreePipits, and Yellow Wagtails passing in twos and threes, in small parties, or it may be in small flocks; and I observe that while some establish themselves in the neighbourhood, others pass on. Or I watch Herring-Gulls returning to the breeding station at Bolt Head, an endless stream of individuals coming from the east as far as eye can reach; following them for some miles inland I see them still, first as specks upon the horizon, then passing beside me as they beat their way slowly against the strong south-westerly winds, and finally disappearing from view in the direction of the cliffs. Or again. I watch Buntings and Finches deserting the flock and seeking stations in the marsh, or amongst the furze-bushes on the common, or in the spinneys. In each case the proximate end of the behaviour is alike—wherein then lies the difference? Only in the distance which separates the territory from the area in which the birds formerly associated. And intermediate between the extremes. I doubt not, if we bad a sufficient body of observations, that we should ​find numerous gradations, the lesser merging step by step into the greater. Is the Swallow a migrant and the Herring-Gull not; is the Tree-Pipit a migrant and the Bunting not; must a bird cross many miles of sea or of land before it can be considered a migrant; is the length of the distance traversed a criterion of migration? Surely not. The distance traversed is merely a collateral consequence of the process as a whole.
The annual life-history of a bird presents, as we have seen, two distinct phases—the one in which the individual dominates the situation, the other in which it is subordinated to the welfare of the community. Let us take these two phases separately and endeavour to see how they may have influenced the seasonal movements; and first let us take the more important of the two, namely that one which is directly concerned in the continuance of the race. In this phase we must consider the three factors to which allusion has already been made:—(1) the internal impulse, (2) the innate ability to return to the former breeding ground, (3) the conditions in the external environment. These three work in close relation and, as I shall endeavour to show, lead to important results.
(1) If there were nothing in the inherited nature beyond an impulse to seek the breeding ground, if, that is to say, when the appropriate locality were reached, the bird took no further interest in the developing situation, the attainment of reproduction would become largely a ​matter of chance. A male in a congested district, having no incentive to seek fresh ground, would remain inactive until a female happened to cross its path and stimulate its sexual impulse, when its activity would take another form. Hence some districts would be over-populated, whilst others would remain unexplored. But the system of reproduction does not consist merely of a search for the breeding ground, and of the discharge of the sexual function; it is a much more complex business, yet withal more complete. Nothing is left to chance; the end is attained step by step; and each successive stage marks the appearance of some specific factor which contributes towards the success of the whole. We start with the appropriate organic condition under which, when adequate stimulation is provided, the disposition to secure a territory comes into functional activity. Within the field of this disposition we can distinguish certain specific impulses. In sequential order we have the impulse to seek the breeding ground; the appropriate situation which gives rise to an impulse to dwell in it; and the act of establishment which supplies the condition under which the impulse to drive away intruders is rendered susceptible to stimulation. Grouping these impulses, for the convenience of treatment, under one general heading. I speak of an impulse to seek isolation. It implies some kind of action with some kind of change as its correlated effect; and from it there flows ​a ceaseless energy directed towards a definite end which for us, who can perceive its prospective value, is isolation in an appropriate environment. The emphasis here is on "isolation," for it involves competition, and there cannot be competition without some change in the relative positions occupied by different individuals; so that in each recurring season there will be not only a re-arrangement of ground formerly occupied but an arrangement of ground formerly deserted.
(2) That the older birds return to the locality wherein they had formerly reared offspring, and the younger to the neighbourhood of their birthplace, was always deemed probable. But in recent years evidence which cannot be rebutted has been supplied by the marking of birds. This evidence, details of which can be found in the summary of results published annually by Mr Witherby in British Birds, demonstrates that the adult frequently returns not only to the same locality in which it formerly bred, but even to the same station; that it does so year after year; that this mode of behaviour is not peculiar to one sex; and that many of the young breed in the locality in which they were reared. Such being well-established facts, we can infer the existence of an innate ability to revisit the place wherein the enjoyment of breeding, or of birth, had formerly been experienced. Of its nature we know little or nothing. It would almost seem as if there must be some recollection of past ​enjoyment, but all that can be definitely asserted is—that past experience somehow becomes ingrained in the life of the individual and determines present behaviour. What, however, is of importance to us at the moment is not the ad hoc nature of the bird, but the biological consequences to which the behaviour leads. For if, on the average, individuals return to their former haunts, it follows that the annual dispersion will not be merely a repetition in this season of that which had occurred in a previous one, but that the little added this year will become the basis for further additions in the next. The innate ability is handed down from generation to generation, and, in so far as it contributes to success, is fostered and developed by selection; and the modifications of behaviour to which it leads, since the results of prior process in the parent persist as the basis and starting-point of subsequent process in the offspring may in a sense also be said to be handed down.
(3) The conditions in the external world may be organic or inorganic. By organic I mean the conditions which depend upon the number of competitors or enemies by which a bird is surrounded. The competitors may include other species which require a similar environment; and the enemies, species which prey upon it, or animals which take its young or its eggs. They vary in different seasons, in different districts, and in nature and extent—the success of one species leads to the failure of another, and the ​multiplication of the Jay or of the Magpie robs us of many a songster.
By inorganic I refer to the changes in the climate and in the surface of the earth. The nourishment of the young depends upon a regular supply of food, and the supply of food depends upon the climate which alters in different periods; in one decade the temperature falls below, whilst in another it rises above, the normal, and, as the insect life fluctuates, so there is fluctuation in the bird population. The changes in the surface of the earth are manifold. Little by little the alder (Alnus glutinosa) overspreads the marsh. Young shoots spring up here and there, in a few years grow into bushes, and in a few more years are trees; and the dense masses of rush which seemed to choke their growth, yielding their position of importance, slowly disappear. And where formerly the Orchis latifolia, Orchis mascula, and Juncus communis grew in mingled confusion, nothing but water, moss, and the spreading roots of alder cover the ground. As the rush disappears, many birds that for generations have inhabited that marsh must seek accommodation elsewhere. Ancient breeding haunts thus disappear, new ones come into being, and even those which appear to be permanent are almost imperceptibly changing.
Now the bird inherits a nervous system, which works under internal excitation and external stimulation. Given the appropriate organic condition and adequate stimulation. ​and the impulse to seek isolation comes into functional activity. What the organic condition is and how it arises we do not exactly know; all we know is that organic changes do take place in the breeding season, that these changes profoundly modify character, and that they correspond with the seasonal growth of the sexual organs. And with regard;to the question of stimulation, we have again to confess to much ignorance, although certain facts are presented to observation which seem to indicate the direction in which the stimulus lies. For example, it is well known that abnormal climatic conditions influence behaviour; we see migrants retracing their flight along the very course they travelled a short time previously—driven headlong by the blizzard, that at least is what we say. But if the wind, instead of being cold and from the north, is warm and from the west, do they retrace their flight? I have not found it so. And if there be no wind and the temperature is low, are they still affected? Again. I have not found it so. When, as we commonly say, they fly before the storm, some change takes place in their organic complex, some new impulse receives stimulation or the former one lacks it. If, after Lapwings have established themselves in their territories, the weather becomes exceptionally severe, the birds collect together again in flocks and revert to their winter routine; and under similar circumstances. Buntings fail to sing and temporarily desert their territories. In such cases it is clear that ​the impulse to seek isolation ceases for a time to dominate the situation. The inference, therefore, is that atmospheric changes bear some relation to the functioning of the instinct; but whether it be temperature, or humidity, or the direction and velocity of the wind, or a combination of two or more of these factors that supplies the stimulus, we cannot tell.
The appropriate organic condition and the stimulus have then still to be determined, and we must pursue our inquiry from the point at which the impulse comes into functional activity. We will take a simple case, and one free from complication.
Let us suppose that there is an area bereft of bird life, if it can be so imagined, but in proximity to other inhabited areas. Into this area, whilst in search of isolation, let us imagine that a Yellow Bunting finds its way. After the manner of its race it establishes a territory and occupies, let us say at a low computation, half an acre of ground. It then obtains a mate, breeds, and rears offspring, two of which we will assume are males. Reproduction ended, the birds desert the area, and in the following spring, when the impulse again asserts itself, parents and offspring seek again their former haunts. We now have three males, each of which occupies half an acre, and each of which rears two offspring—that is the position at the close of the second year. In the third year the number will have increased to nine and the area occupied to 4½ acres; and so on in succeeding years, until by the beginning of ​the eleventh year, we have 59,048 Yellow Buntings occupying 29,524½ acres or 46 square miles. This, then, will be the result of the operation of the impulse, providing that all the individuals survive and that no complications supervene.
But of course complications are numerous, some of which retard while others accelerate the rate of expansion. These complications arise from various sources—in the first place from natural enemies which prey upon the birds or upon their eggs; in the next place from climate which, if it happens to be unfavourable, may mean that food is scarce and that only a small percentage of the young survive; and lastly from rivals—and by rivals I mean closely related forms that require a similar station and similar food—which, by occupying available ground, may check expansion, or, by forcing a continuation of the search, may widen it.
Now when individuals fail as many do fail in their initial attempt to secure territory, the activity of the impulse still persists, and there is no control over the direction in which the bird continues to wander whilst in search of its end. Some therefore seek in this direction, others in that; some wander inwards into inhabited areas and fail to find accommodation, or, according to the relative strength of their impulse, perhaps succeed and so set free a new competitor, others wander outwards into country uninhabited by the species. These latter we will call "pioneers." They may find accommoda​tion within a comparatively short distance of their base, or they may come into competition with rivals and fail, not necessarily on account of any congenital weakness of ability, but because, being warned by an alien song, they may be precluded from coming into contact with just the environing conditions which can supply the stimulus and allow behaviour to run its further course—and so be obliged to extend their search into remoter districts. But it must not be overlooked that they will be placed in a most advantageous position so far as the attainment of reproduction is concerned. In their search for territory they will meet with little opposition and will be free to select whatsoever ground they will; and be free also from intrusion by neighbouring males, which is so frequent in occurrence and continues for so long in congested areas. Moreover, in thinly populated districts, the pressure upon the available means of support will not be so great, neither will natural enemies be so plentiful; and since the offspring, guided by prior experience, return to the neighbourhood of their birthplace, the advantages thus gained will be shared by the succeeding generation. It follows, then, that the range of a species will not always be continuous, will not, that is to say, proceed by a series of successive steps, but that sometimes in this direction and at other times in that, the chain of territories will be interrupted and different individuals separated by distances of greater or lesser ​extent. New colonies will thus come into being; and as the unlimited increase of the population over limited areas gradually reintroduces into them the struggle for territory, new centres of distribution, where the process will repeat itself and from which expansion will proceed afresh, will be formed. Hence, though it is clearly impossible for the progeny of one pair of Yellow Buntings to overspread the whole of the 46 square miles, it is by no means impossible for the limits of their range to exceed even those limits within the eleven years.
To sum up our knowledge regarding this phase. Of the organic condition which renders the impulse responsive to stimulation we know very little; and though certain facts of observation seem to indicate the direction in which the stimulus is to be found, we must here again confess to much ignorance. So far as can be seen, however, the impulse to seek isolation with its correlative territory, leads to constant modification in the breeding range of most species. The occupation of the small space of ground which each individual requires, the extent of which has been gradually adjusted to suit the needs of different species, results in expansion not only in one direction but in every direction, and not only in one season but in every season. And if there were no complications in the external world this expansion would proceed, as we have seen, with astonishing rapidity. But complications, some of which are favourable and others unfavourable, are numerous, and it is ​difficult to estimate their importance or to indicate their precise effect; the former, however, accelerate the rate of expansion, whilst the latter retard it. Those individuals that wander outwards and seek territory on the outskirts of the range we have called "pioneers." They will have advantages over others that, wandering inwards, seek isolation in congested districts, and will succeed where the latter fail; and since there is in the young an innate ability to return to the district wherein they were reared, the advantages so gained may be said to be handed on from generation to generation.
Let us now turn to the contra-phase, and endeavour to ascertain whether the gregarious instinct bears any relation to the seasonal desertion of the breeding ground. The conclusion at which we have already arrived regarding this instinct is that it forms part of the inherited nature of most species; that its functioning is suppressed when a bird is actually in occupation of a territory; and that it is serviceable in promoting the welfare of the individual. We cannot of course observe the instinct. What we observe, when reproduction is ended, is a change in the relations of different individuals; instead of arousing mutual hostility, they attract one another, from which we infer the existence of something which determines their conduct, and this "something" we speak of as an instinct.
To what does this change lead? Let us suppose that there is an area inhabited by one ​species; that the number of inhabitants has reached the maximum that the means of sustenance will allow; and that the season of reproduction is drawing to a close. The position will then be as follows. All the available breeding ground is divided into territories; each territory is occupied by one unit, the family, and each individual is able to fend for itself; changes both internal and external begin to take place, the gregarious instinct comes into functional operation, and the supply of food diminishes—that roughly is the position. The internal factor operates so that the sight of this individual or the call of that, instead of evoking hostility as heretofore and keeping different units apart, proves now an irresistible attraction; so that in place of a number of individuals evenly dispersed over the whole of this area, a small number of flocks of various dimensions are stationed at certain points, which points are determined partly by experience, partly by the supply of food, and partly by accident. This implies for each individual some movement in some direction. But since the population of this imaginary area has reached the maximum, and the supply of food, though limited in distribution, is nevertheless plentiful, such movements will be irregular and will proceed in no definite direction.
Now let us suppose that the breeding range extends and that fresh ground is occupied by pioneers. When reproduction and the rearing of broods are ended and the gregarious instinct ​becomes dominant, these pioneers, or at least some of them, will revisit the area wherein formerly they associated with companions. Their offspring, however, though they will have the inherited impulse and the innate tendency, will not have the experience; how then will they behave? There can be no doubt that some will accompany the older birds, and, being led by them, will share the experience of a former generation; nor any question that others will collect together in the neighbourhood of their birthplace and, if their impulse is satisfied, will remain there so long as food is to be found. Thus the gregarious instinct, working in close relation with acquired experience, will on the one hand lead to the formation of organised movements in certain directions, whilst on the other it will lead to the formation of new areas of association which will follow in the wake of the expansion.
We have assumed, in the imaginary case which we have just taken, that the conditions in the external world are such as enable the birds to endure throughout the year—in short, that there are no complications regarding the supply of food. But we must bear in mind that so long as conditions are favourable during the period of reproduction, which is of short duration, the breeding range can continue to expand,, and that therefore, in the course of centuries, regions will come to be occupied wherein, owing to alternations of climate or physical changes in the surface of the earth, ​food will be impossible, or at any rate difficult to obtain at certain seasons. Hence th^re will come a time when the area of association ceases to follow in the wake of the expansion, and the breeding area begins to diverge from the subsistence area.
How, then, is the gulf between these two areas to be bridged? We can of course say that those individuals which, in virtue of some slight variation of hereditary tendency, return to regions where food is plentiful will survive; whilst others, less well endowed, will perish. We can state the position in some such general terms, and doubtless there would be truth in the statement, but it does not carry us far; we wish to know more of the nature of the tendency, and of the manner in which it has evolved. Well now, in this new situation which arises, two things are apparent—that the struggle for existence becomes a struggle for the means of subsistence, and that anything in the inherited constitution of the bird which can be organised to subserve the biological end in view becomes of selection value. So long as food can always be procured in the new areas of association, the individuals that behave in accordance with ancestral routine gain thereby no particular advantage; but directly the breeding range extends into regions where the supply fluctuates, traditional experience becomes a factor in survival, and those individuals that come under its influence will, on the average, be more likely to endure and so ​to procreate their kind and maintain the tradition. Let it once be granted that there is an innate capacity to retain in later phases of routine the experience gained in earlier phases, and it is difficult to see how traditional guidance can be refused recognition as a factor in the developing situation. But only a factor, and by no means the most important one; for observation has shown that the young are capable of performing the return journey without guidance. Something therefore is inherited, some impulse which comes into functional activity at a specified time, and leads the bird to set forth in a given direction.
There are no grounds for supposing that the experience of one generation forms any part of the hereditary equipment of subsequent generations. In what direction then are we to look for the congenital factor? What is given is an inherited tendency to co-operation and mutual help, and an innate capacity to make use of the results of experience. The inherited tendency, as we have seen, leads on the one hand to the formation of new areas of association, whilst on the other, since it is the means of bringing isolated individuals into contact, it leads to experience being handed on from generation to generation, which, in its turn, results in a certain amount of backward movement along the line of expansion. It forms part of the hereditary equipment of many species, and is serviceable in promoting the welfare of the individual. Moreover, there is ​reason to believe that its origin dates back to an early period in the evolution of the higher forms of life; and if in" the subsequent course of evolution it could have been so organised as to serve a double purpose, so much the more reason would there have been for its survival. In what does the instinct consist? Is it merely that the sight of this individual or the call of that proves at some particular moment an irresistible attraction, or does the appropriate organic condition give rise, as is generally supposed, to some preceding state of uneasiness? In the former case, the temporarily isolated individual or colony would have but little chance of sharing in the benefits which mutual association confers upon the associates; in the latter, the feeling of discomfort would lead to restlessness, and would thus bring the bird into touch with the environing circumstances under which instinctive behaviour could run its further course. So that it is probable that the movements of each individual, prior to its becoming a unit in the flock, are not accidental but are determined in some measure by racial preparation.
Now if the fundamental assumption of the doctrine of the struggle for existence be true, the gregarious instinct will not be quite alike in all the members of different broods, nor even in each member of the same brood; that is, variation will occur in all possible directions. And we shall not. I think, exceed the limits of probability if we assume that different individuals ​vary in the persistency with which they strive to attain their unknown end, and in the direction in which they travel in pursuit of it. So that in each generation they will fall into three classes: (1) those which are inert, (2) those which wander along the line of expansion, (3) those which wander in other directions. If then the struggle for life at this particular juncture in the evolution of the breeding range is a struggle for the means of subsistence, the members of these three classes will not be in a like satisfactory position so far as the competition for food is concerned. Those in the first class—i.e., those in which the activity feelings are weak—will neither gain the benefits which arise from mutual help, nor will they have much prospect of enduring through the season of scarcity. Those in the third class will, it is true, derive some assistance one from another, and so be in a better position to discover what food may be available; but inasmuch as they will remain in regions where the climate alternates' and the supply of food is liable to fall below the minimum required, the chances are that a high percentage will fail in the struggle for existence. We come now to those in the second class, and it is upon them that I wish more particularly to focus attention. The initial movement in their case will be in the direction from which outward expansion has all along taken place. Within a comparatively short distance they will reach districts where the species is plentiful, and here, associating with others that have some traditional ​experience, they will be guided by them and will find themselves in regions where food is plentiful. Hence in each generation those will survive that, owing to some congenital variation of their instinct, seek satisfaction for their impulse in a direction which brings them under the influence of tradition. And though at first but slight and not in themselves of survival value, such variations, since they coincide with modifications of behaviour due to acquired experience, will be preserved and in the process of time so accumulated as to be capable of determining the direction and extent of the movement.
But the young Cuckoo deserts this country, many weeks after its parents, and there is no reason to suppose that it lives in society when eventually its destination is reached; and the young Falcon passes to the south, and is certainly not gregarious—how then can we explain their behaviour in terms of something which they show no signs of possessing? I do not wish to make light of a difficulty which admittedly, at first sight, is a grave objection to the view that the gregarious instinct has been operative in the manner here claimed for it. It must, however, be borne in mind that this instinct, though originally developed to serve the purpose of mutual protection, supplies thie material upon which evolution works when the extension of the breeding range creates a situation requiring readjustment on the part of the organism to new conditions of life; and that ​those variations which can be so modified as to be in useful relation to the new environmental circumstances are seized upon by natural selection and, being transmitted, form the foundation of a specific inherited response, no longer dependent upon, though operating in close relation with the primitive response whence originally it sprang. Thus the primordial instinct becomes so organised as to serve a secondary purpose, that of rendering secure a means of access to a certain food supply. In the course of evolution species were bound to arise which, owing to some peculiar conditions, derived greater advantage from living solitary than from living in society. Does it then follow, because such species manifest no inclination to live in society, that the instinct never has played any part in their lives? Or because the primary purpose has lapsed, does it follow that the secondary no longer exists?
Let me recapitulate the principal considerations which I have discussed in this chapter.
Though I have been advancing a theory, and though I have taken much for granted, yet it will. I think, be admitted that both the theory and what has been taken for granted rest on observational grounds. As our starting-point we have a bird whose inherited nature alternates according to the season, and in whose nature we can distinguish two contra-phases—the one to live in society, the other to live solitary. While both have their 'part to play in furthering the ​life of the individual, for biological interpretation there is only one end, the prospective value of which is the continuance of the race. We may say that the latter phase is the more important of the two because it is directly concerned with reproduction. But we shall make a great mistake if we attach peculiar importance to one phase, or to one mode of behaviour within that phase, or to one action within that mode of behaviour; for if there is one thing certain it is that the whole is an inter-related whole in which each part depends for its success upon that which precedes it.
In that phase in which the territory is the central feature of the situation, the struggle for existence is in operation in its acutest form; all the congenital and acquired capacities of the bird—pugnacity, song, capacity to utilise in later phases the experience gained in prior phases, all these are organised to subserve an end—a proximate end—which in its simplest terms may be described as "isolation." Isolation is then the first step in the process of reproduction, and any individual that fails to make it good, fails to procreate its kind. But isolation implies separation, and the degree of separation varies in different species, from the few square feet of cliff required by the Guillemot to the few square miles of barren moor over which the Peregrine exercises dominion. One species must occupy sufficient ground to enable it to secure food for its young; another requires sufficient, but no more, upon which to deposit its egg; and ​a third must secure a position for its nest within the community. Hence it follows that the degree of separation varies with the conditions of existence. Since, however, the conditions in the external world are constantly changing according to the relative abundance or scarcity of enemies, the rise or fall of rivals, the physical changes in the earth's surface, and the alterations of climate, it is clear that isolation can only be obtained with difficulty, and that the competition for it must be severe. Some individuals therefore fail to breed, whilst others, perhaps because their impulse is stronger, persevere and seek stations elsewhere. What are their prospects of finding them? By extending the field of their activities, they will wander into districts remote from the scene of competition, districts where not only food is plentiful but where enemies and rivals are scarce; and to these pioneers, if to any, success in reproduction will most certainly be assured. But not only is it they who will benefit; their offspring also, when the time comes for them to take their part in the maintenance of the race, will share in the success of their parents, for even though they may not escape competition from individuals of closely related forms, they will meet with but little from those of their own kind. Now species which live throughout the year in the vicinity of their territory are comparatively few, the majority are obliged to wander in search of food so soon as reproduction is ended, and their behaviour is determined not only by its ​abundance or scarcity, but also by the powerful gregarious impulse which waxes in proportion as the instincts connected with reproduction wane. If, then, when the sexual instinct again becomes predominant, the experience of the former season nowise affects their movements, little or no progress will be made in the expansion of the range. But just as a certain entrance into the bush and pathway through it, when once made use of in the process of building, becomes so firmly established as to form the sole highway to and from the nest, so likewise, when the impulse to seek isolation repeats itself, the bird is constrained to seek the neighbourhood wherein it had experienced the enjoyment of breeding or of birth. Thus the little that is added one year becomes the basis for further additions in the next, and new centres of distribution are continually being formed from which expansion proceeds anew.
Now as the range gradually extends into regions where the climate alternates and food at certain seasons is consequently scarce, the distance between the customary area of association and that of reproduction must perforce widen. The question then arises: How will the young that have no experience find their way to regions wherein they can endure? The forces which may have been organised to subserve the end in view are three: (1) Acquired experience, (2) tradition, (3) the gregarious instinct. The pioneer that carries the range a little further forward starts from a base where it has associated ​with companions and found food plentiful; and when the impulse to live in society again asserts itself, it not only repeats its former experience but hands on the habit thus acquired to those of the next generation that happen to accompany it. Granting, however, that by successive increments in the distance traversed, traditional guidance may in time accomplish much, it cannot account for all the known facts, it cannot at any rate explain the fact that in some cases the inexperienced offspring finds its way to the food area without guidance. Something, therefore, is inherited. And my suggestion is this: That the gregarious instinct, the ancient origin of which we can infer from its manifestation in so many and diverse forms of life, supplies the material upon which evolution works; that variations of the initial impulse, at first sight and not in themselves of selection value, in so far as they coincide in direction with modifications of procedure due to experience or tradition, are preserved; and that, in the process of time, they are so accumulated as to form a specific congenital endowment determining a definite mode of behaviour.