Territory in Bird Life

by Henry Eliot Howard

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.