BRITISH WARBLERS
imitated of which we have been speaking, and which cannot well supply the conditions necessary to produce differential effects upon the female. For, however broadly we interpret this view, however confidently we assert that imitations, broadly speaking, create a more effective pairing situation, yet inasmuch as of two individuals it will always be the one that has acquired more numerous or more perfect imitations which will attain to reproduction, there must result a constant tendency towards variation which can scarcely make for the similarity that we observe. If we had sufficient knowledge of the vocal powers of those species in which the faculty was highly developed, we might possibly find that it always followed certain definite lines, that it was, so to speak, the outcome of a certain type of vocal development. A song, for instance, in which articulation is conspicuous and the utterances distinct may lend itself more readily to imitation than one in which the flow of phrases is more continuous. If this were really the case, the tendency to imitate song need not necessarily be of service to the individual, but only the result of a definite type of instrument, and on this view the similarity referred to would become more intelligible. But evidence on many points is still needed. We require information concerning the behaviour of the males in their winter home. Do they there sing and copy indigenous species? We need information concerning the course of and the time occupied in the migratory journey, and definite knowledge as to whether each individual male, upon arrival at its breeding home, introduces imitations into its song, and to what extent variation occurs in the quality and quantity of such imitations. There is room for much observation here.
Of the call notes it is necessary to say but little. The most common one is not an unmusical sound, and is similar in some respects to the corresponding note of both the Reed and Sedge Warblers, but rather more distinctive than either of these. It is uttered at various times and by both sexes, and can be
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