CHAPTER VIII 



VARIATION IN BIRD-VOICES : ITS CAUSES 



AND EFFECTS 



WE may safely lay it down as an axiom that vocal 

 utterance is always subject to variation. Even in the 

 newt and the snake such a variation occurs, though 

 in the former it is an accidental result of altering 

 degrees of violence in the contortion of the body, 

 and in the latter it indicates varying intensity of 

 fear or of hate. There is not much variation in the 

 cries of the youngest birds ; yet even here it occurs, 

 and from causes analogous to those observable in 

 the reptile, for when the suppliant has been satisfied 

 he ceases to exclaim, and when very hungry he 

 is most persistent in his cries. The same pheno- 

 mena produce in him the same results, until he 

 becomes sufficiently independent to abandon the 

 use of cries for food, and then he simply calls occa- 

 sionally for the companionship or aid of his parents. 



