BRITISH WARBLERS
be compared to a flute, the inferior song lacking the fulness or richness of tone, and not being as powerful; and there is also this difference, that that part of the song which contains these notes is not of the same length nor so frequently produced. The inferior song I have sometimes traced to males with dull, undeveloped plumage, but the purer song, which, though not frequent, is not of uncommon occurrence, is invariably the production of males with highly developed colouring and plumage. Between these two extremes the gradations are numerous. It is difficult to prove that such gradations are due to different stages of growth, owing to the impossibility of keeping the same birds under observation from year to year. Nor is it likely that, under confinement, the conditions could be made sufficiently natural for the results, to be reliable. But the evidence rather leads to the conclusion that a gradual strengthening and perfecting of the vocal organs does take place, though whether as a result of practice alone, or from a combination of causes, it is impossible to say; and this conclusion gains considerable support from the fact that in the case of other species with highly developed vocal powers, such as the Blackbird (Turdus merula), a development can undoubtedly be traced from year to year.
(2) The deterioration of the song is an interesting phenomenon. When the males arrive in spring their vocal powers are at their best; this condition continues until mated, when a gradual deterioration takes place. About the middle of May it is not unusual to hear them commence their song by a single note repeated three times in succession. The song itself now lacks vigour, is shorter and more confused, the beautiful part of it is produced less frequently, and the notes themselves lose much of their original sweetness. I once heard a male that sang beautifully earlier in the spring, repeat continually and almost perfectly the first line of the song "Pop goes the Weasel." Proof that sexual intercourse is the direct cause is lacking. But bearing in mind these
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