142 EVOLUTION OF BIRD-SONG 



The yellow bunting, chaffinch, willow -warbler, 

 and chiffchaff, for instance, would by most people 

 be considered to sing unvarying songs ; but some 

 evidence to the contrary will shortly be mentioned. 

 The most general variations are in relation to the 

 extent or rapidity with which an exclamation is 

 uttered ; less general methods occur in intervals 

 of musical pitch ; still less frequent is the acquire- 

 ment of a note or tone quite distinct from that of 

 the ordinary song of a species ; yet even this last is 

 a fairly wide-spread feature. Variation in the song 

 of adult birds is not necessarily induced by erotism, 

 for it occurs in birds which never see a mate ; but 

 there can be no doubt that the presence of a mate, or 

 the desire for such presence at the breeding season, 

 is the great stimulus which causes efforts towards 

 extravagant song, either in the direction of a pro- 

 longed or a vehement utterance, or to the production 

 of unusual cries. I have several times seen starlings 

 utter their alarm -cries, apparently as a means of 

 attracting the attention of the female bird ; and 

 sometimes execute an extraordinary devious flight 

 a succession of swoops and rushes, seemingly in- 

 tended to excite the attention of other starlings, 

 near which they had been singing. I have twice 

 observed precisely the same behaviour, and in the 



