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 pro- 

 duced. 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 conclu- 

 sion 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 pheno- 

 menon. 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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