CHAPTER VI
THE WARFARE BETWEEN DIFFERENT SPECIES AND ITS RELATION TO THE TERRITORY
We have now considered the various ways in which the territory is useful in furthering the life of the individual. We have seen that, in some cases, there is competition for stations where the egg or eggs can be deposited and incubated in safety; that, in others, there is competition for stations capable of furnishing an adequate supply of food for the young; and that the establishment of "territories" not only renders the attainment of reproduction for the individual secure, but serves so to regulate the distribution of pairs that the maximum number can be accommodated in the minimum area. This being so, the question arises as to whether competition for territory is strictly limited to individuals of the same species, or whether it may not occur also between different kinds of birds, providing always that similar conditions of existence are required. First of all I shall relate a number of facts which will serve to show the nature and extent of the warfare, and I shall then give the reasons which lead me to believe that the fighting not only bears some relation to the "territory," but that it is an important factor in contributing to the attainment of that which for biological interpretation is the end for which the whole territorial system has been evolved.
Those who have studied wild life on one of the rocky headlands, which are so numerous round our coasts, will probably be familiar with the rivalry that exists between the Raven and certain birds of prey. Where the Raven finds shelter for its nest, there, too, the Peregrine has its eyrie—and so it happens that these two species are continually at war. Now the warfare occurs not only during the season of reproduction but continues throughout the greater part of the year, and can even be observed in the late summer or early autumn—the period when we should expect to find the instinct least susceptible to appropriate stimulation. But it is of a more determined kind early in the spring, and it is then that we often witness those remarkable exhibitions of flight, the skill of which excites our admiration. The Falcon rises above the Raven, stoops at it, and when it seems no longer possible for a collision to be avoided, or, one would imagine, for the Raven to escape destruction, the Raven skilfully turns upon its back and momentarily faces its opponent, and the Falcon with equal skill changes its course, passing upwards and away. The attack, however, is soon repeated, and though no collision may actually take place, yet the fact that the Raven, when it turns to face its adversary, is obliged to drop the stick which it carries, is not only an indication of the character of the struggle, but it shows that a definite end is gained—that the efforts of the Raven to build in that particular locality are hampered. But the Falcon is not the only enemy that the Raven has to face; Buzzards are just as intolerant of the presence of Ravens in their neighbourhood as the Ravens are of them, and consequently there is incessant quarrelling wherever the same locality is inhabited. As a rule, the fighting occurs whilst the birds are on the wing; the Buzzard rises to a considerable height, and, closing its wings, stoops at the Raven below, and when within a short distance of its adversary, swerves upwards and gains a position from which it can again attack. The Buzzard, however, is by no means always the aggressor; I have watched one so persistently harassed by a Raven that at length it left the rock upon which it was resting and disappeared from view, still followed by its rival. Thus it seems as if they were evenly matched, and, when they occupy the same locality, it is interesting to notice how the initiative passes from the one to the other according to the position occupied by the birds in their respective territories.
That there is constant warfare between the Green Woodpecker and the Starling is well known, the purpose of the Starling being to gain possession of the hole which the Woodpecker with much skill has drilled for itself. As far as my experience goes, the Starling is always the aggressor, and there is only too good reason to fear that, in the course of time, the Green Woodpecker will disappear as a result of the greater fertility and tenacity of its enemy. The Martin suffers a similar kind of persecution from the House-Sparrow, and here again there is reason to believe that the greater virility of the Sparrow will hasten the extinction of its rival. In cases of this description the purpose of the fighting is clear, and one can understand why such divergent species should be hostile to one another; yet others, equally remote in the scale of nature, are hostile when no such ostensible reason can be assigned for their hostility. Few birds are more pugnacious than the Moor-Hen, and the determined manner in which different individuals fight with one another is notorious. But the intolerance it displays towards other species is no less remarkable, and its pugnacious instinct seems to be peculiarly susceptible to stimulation by different individuals belonging to widely divergent forms. At one moment a Lapwing may be attacked, at another a Thrush or a Starling, harmless strangers that have approached the pool to drink; even a Water-Rail, as it threads its way through the rushes, may fail to escape detection; and, which is still more curious, a covey of Partridges will evoke response if they approach the pool too closely.
Here is a curious instance of apparent waste of energy. A pair of Magpies built their nest in an ilex tree. Early one morning there was a commotion in the tree, much flapping of wings and a medley of sounds which told of large birds engaged in a struggle—the Magpies were attacking a pair of Wood-Pigeons. There was no question as to the genuineness of the struggle, nor any doubt as to the proximate end for which the Magpies were striving, for their efforts continued so long as the Wood-Pigeons remained in the tree, and only ceased when they had succeeded in driving them away.