Territory in Bird Life

by Henry Eliot Howard

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

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

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

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

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

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

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

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

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

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