BRITISH WARBLERS
only to watch the oak trees when the larvæ of the Tortrix viridana are hatched to see the truth of this, for quantities of Blackcaps, Garden Warblers, Willow and Wood Warblers, and Chiff-Chaffs there find a living, or the alders adjoining some reed bed which, when the aphides are swarming, attract numbers of Reed and Sedge Warblers, Chiff-Chaffs, Willow and Garden Warblers. There remains the emotional behaviour upon which opinion is so divided. The continuity which we can almost trace in structure and instinct fails us here, and in place of it we are faced with facts of that contradictory kind which nature so often reveals. Those two small birds, perched on the same branch within a few feet of one another, flapping their partially expanded wings, we know at once to be a pair of Willow Warblers, for the Chiff-Chaff behaves differently. That bird, which rising from the hedge dances in the air as if suspended by an elastic thread, we recognise without difficulty, since its nearest of kin the Lesser Whitethroat has other means of expressing its emotions. And similarly we distinguish the Blackcap from the Garden Warbler, for each during times of excitement spreads out its wings and tail and raises its feathers in certain ways peculiar to it. Yet we cannot by such means distinguish between the Grasshopper and Savi's Warblers; their actions are identical; they spread and move their wings similarly, and if it were not for the difference in colour and in the pattern on the feathers, or the much more musical trill of the one, no one could say with certainty whether the bird he was observing belonged to this or to that species. Here is a somewhat different case. Let us imagine ourselves in the centre of some reed bed towards the end of May. Reed Warblers are all around. Beside us is a male, evidently sexually excited, uttering its metallic sounding song, and whilst doing so jerking its wings slightly outwards from the shoulder. Leaving it we enter an osier bed from which a more beautiful, more highly developed song is proceeding. To all appearances the owner of the voice is a Reed Warbler; it too is excited, singing to the
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