Territory in Bird Life

by Henry Eliot Howard

In giving an account of the domestic economy of the Carrion-Crow, Mr. Edmund Selous refers to the hostility between this bird and the Magpie. "About a week ago," he says,[6] "I saw a Crow busily engaged in chasing away several Magpies, not only from three or four tall slender trees close together, in one of which it had its nest, but also from various other trees, not far off, round about. In this the Crow had a good deal of trouble, as the Magpies were always returning. After a time it was joined by another crow, which however did not take so active a part in the drama, nor did I see either of the two actually go to the nest, though I could only explain their action by supposing it was their own. This morning I saw the same thing reversed, for a pair of Magpies, with an undoubted nest, kept attacking a Crow that insisted on settling in one of a row of trees—also tall and slender—in which it was placed. Both were equally persevering—the Crow, though often chased away, always returning, and settling generally in the last tree of the row, where he would be left alone sometimes for a minute or two, but before long one of the Magpies flew at him, and put him to flight. The Crow defended itself, but not, it would seem, very successfully, and in the last attack upon him, made, with great spirit, in the air, a large black feather floated to the ground, which I made no doubt was his. Yet this did not drive him from the trees, and it was only on my approaching nearer that he finally left them. Thus we see that both species look upon the approach of the other to within a moderate distance of their nest as an intrusion."

That the Rook suffers persecution from the Carrion-Crow is a well-established fact, and there is reason to believe that it has another dangerous enemy in the Hooded Crow. According to the late Mr. Ussher, Choughs will attack both Hooded Crows and Ravens. "I once saw," he says, "two Choughs energetically attacking a pair of Ravens; they shot up into the air and darted down on the latter, whose heavy flight made them helpless against their agile tormentors."

Birds of prey are often hostile to one another. The Merlin is exceptionally pugnacious, and its boldness in attacking intruders is well known. When, for example, a Kestrel approaches its territory, it leaves the tree, bush, or rock upon which it was resting, utters its characteristic cry, and soars rapidly upwards; then, rising to a considerable height, it swoops down upon the Kestrel, and by alternately stooping at and chasing its opponent, drives it away from the immediate neighbourhood.

What we have, then, to consider is, Do these battles between different species contribute towards the attainment of the end for which the whole territorial system has been evolved?

Let us take the individual and see whether we can establish any relation between the hostility it displays towards members of other species and its general disposition to secure a territory. We must remember that a male can have no knowledge of the prospective value of its behaviour, nor is it likely that it has any ulterior purpose in ejecting other males, beyond the pleasure it derives from satisfying its impulse to do so. The proximate end of its behaviour is to attack, nothing more, and this, of course, it can only do just in so far as the intruder evokes the appropriate instinct.

Now the arguments we shall employ will, on the whole, be similar to those which we made use of in the second chapter, wherein we attempted to ascertain the conditions under which a male becomes intolerant of other males of its own species, and examined more especially the claims of the "territory" as opposed to those of the "female." But here we start on firmer ground, because the one factor which introduced an element of uncertainty—the female—can be definitely excluded; at least it seems so to me, for granting even that her presence is the condition under which the pugnacious nature of the male is rendered susceptible to stimulation, it is difficult to see why a male of a different species should supply that stimulus, or what biological purpose could be served by its doing so.

When dealing with the attitude of a male towards others of its kind, we attached considerable significance to the fact that its pugnacious nature gained or lost susceptibility according to the position which it happened to occupy. We found, it will be remembered, that the same bird that was pugnacious in its own territory took no further interest in its opponent when the boundary was passed; and, moreover, that if it happened to wander into an adjoining one, it made no real effort to defend itself when attacked, but returned forthwith to its own headquarters. It remains to be shown whether the rivalry between different kinds of birds is similarly related to the position which the opponents happen to occupy at the time.

First, then, there is the general consideration, namely, that the enmity occurs for the most part just at the time when the territories are in process of being established. During autumn and winter, many birds of more or less close affinity assemble together in flocks, wherever the supply of food is abundant, and are then not only sociable, but, so there is reason to believe, are mutually helpful both in discovering the necessary means of subsistence which are often none too plentiful, and in affording protection from enemies, which, on the contrary, are often numerous. That the different units of which these flocks are composed should live on amicable terms is therefore as necessary for the welfare of the whole community at this particular season as that the different individuals of the same species should do so. But just as the sociable relations, which obtain between these individuals throughout the winter, undergo a marked change at the commencement of the breeding season, so, too, do different species, which habitually associate together, suddenly become hostile to one another. This change is coincident in time with the rise of the organic condition which leads to the establishment of territories; and the hostility continues, though in diminishing degree, throughout the breeding season, and dies away the following autumn.

For example, different Warblers resort to the elders (Sambucus nigra) in September, and there pass much time feeding on the fruit which is then ripe and often abundant. In the same bush there may be Blackcaps, Garden-Warblers, Whitethroats, and Lesser Whitethroats, some preening their feathers, others searching for the berries, others again, with feathers relaxed, making feeble attempts to sing. Occasionally there may be a scuffle, perhaps between a Blackcap and a Lesser Whitethroat, or between a Garden-Warbler and a Blackcap, but it is of short duration and lacks vigour. Apart, however, from such temporary disturbances, there is no real rupture in their relations, and certainly nothing to lead one to suppose that the bickerings are determined by the functioning of any specific instinct. Yet only a few months previously some of them were constantly at war, and their quarrels betrayed symptoms of great persistence; and if we remember how the observed behaviour of the birds suggests the fact that they were striving to attain something definite, we shall understand the nature and extent of the change, and shall, I fancy, be in a better position to estimate its biological worth at its true value.

We can find many similar examples—flocks are to be found on arable ground, on the water meadows, and on the mud-flats; here different kinds of Thrushes feed on the berries of the yew, there different kinds of Tits travel together in parties; hosts of Finches collect in the hollies to pass the night and Buntings roost together in the gorse; and, in fact, in whatever direction we choose to look in the autumn and winter, we find various birds assembled together and living on amicable terms. All of this changes in the spring, and the relationship undergoes a gradual but noticeable alteration; so much so that whereas the outstanding feature of bird life in the winter is sociability, that of the spring is hostility.