Territory in Bird Life

by Henry Eliot Howard

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 from 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, Tree-Pipits, 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 had 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.