46 E VOL UTION OF BIRD-SONG 



instance of this which has come under my observation 

 occurred when a nightingale allowed me to approach 

 within three feet of him, without interrupting his 

 song he being apparently as ignorant of my 

 presence as of the keen delight I derived from his 

 music. Hunger, again, may induce incessant labour, 

 like that of the woodpeckers ; or watchfulness, with 

 occasional repletion and consequent lethargy, as in 

 the Colymbidae, Raptores, and others, all of which 

 conditions militate against the development of the 

 voice, for they are not consistent with that abundant 

 leisure and contemporaneous vivacity which are 

 necessary to song, either in birds or in man. 



The persistence of certain characteristic cries, 

 including call-notes, among species with extended 

 vocal range, may be accounted for not only by the 

 filial mimicry of the nestlings, and this in many 

 species is very powerful, but also by the importance 

 of the recognition of the notes by the parents, which 

 are generally directed to their fledged young by the 

 cries of the latter ; and we may also suppose that 

 sexual selection has likewise tended to the per- 

 petuation of certain cries, for a female bird would 

 very probably prefer a male whose call-note closely 

 resembled the tone which she, when young, had so 

 often employed to obtain food and protection. 



