Territory in Bird Life

by Henry Eliot Howard

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 there 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 the 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 slight 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.