GARDEN WARBLER
source of irritation, and that the behaviour thereby falls into line with that which is commonplace in the lives of other species, is not without significance. Instances of such antagonism have already been referred to. Whenever two closely allied forms, requiring similar conditions of existence, come into touch with one another, there we so often find evidence of the inevitable struggle for existence. We are thus led back to the question of breeding territory. The Garden Warbler and the Blackcap inhabit a similar environment, require similar food, and, like the Willow Warbler and Chiff-chaff, probably compete with one another; for where both species are abundant, conditions must at one time or another arise which will give a slight advantage to the one that through superiority in some direction is better able to ensure an adequate supply of food for its offspring. I believe, therefore, that these scenes which we witness are but the expression points of this struggle for territory, and further that we can explain in the same way those violent outbursts of song which are coincident with the other manifestations of excitement, and are often the prelude to a conflict. The violent use of the vocal powers when the Garden Warbler and Blackcap come into contact with one another is sometimes ascribed to a cause other than that which we are here suggesting; the song itself is regarded as the principal feature, and an appreciation of their vocal powers by the birds as the stimulus to the outburst. But if song has a meaning in relation to the question of territory, we have in this fact a more simple explanation, the term "song" being held of course to include sounds of all descriptions produced under these peculiar circumstances. From the biological standpoint, there is no difference between the monotonous call uttered by the Green Woodpecker whilst occupying his special tree morning after morning, and the passionate but beautiful outburst of the Blackcap. The song is only one of a series of activities which work towards the goal of reproduction. Through it the females are enabled to recognize and locate
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