VARIATION IN BIRD- VOICES 141 



There is a degree of variation due to physical 

 development, such as one observes in the young of 

 the mute swan and the duck, whose voices change 

 when maturity approaches, in much the same way 

 as human voices change, becoming lower in pitch 

 and altered in tone. During all this period of 

 development, from helpless infancy to capable 

 maturity, variations of vehemence, and the extra- 

 ordinary excitement of certain of the emotions, are 

 frequently inducing proportionately pronounced 

 modifications of the voice. Later on occur those 

 variations which are always traversed before the 

 first complete songs are produced. In some 

 instances the latter are, as I have shown (Chapter 

 V.), mere repetitions of a call - note, displaying 

 variations in pitch ; but in many species, and 

 especially in those popularly considered to be the 

 best singers, songs are developed from exclamations 

 of much wider range ; in fact, the songs of mature 

 birds are not usually attained without varied 

 exercises performed by the young in the course of 

 developing their voices. While close attention is 

 necessary to detect variations in the cries of nestling 

 birds, it is also needed to observe alterations in the 

 songs of many birds which are generally considered to 

 repeat their phrases in the same respective characters. 



