146 EVOLUTION OF BIRD-SONG 



those generally prevalent in their respective species. 

 The following are some instances of variation : The 

 hooded crow is said to vary its tone occasionally, 

 producing two cries the one hoarse, the other shrill 

 (ist ed. Yarrell, vol. ii. p. 85). "In the breeding 

 season it sometimes utters more musical sounds." 

 Darwin remarks that the voice of the common rook 

 is known to alter during the breeding season, and 

 that it " is therefore in some way sexual " (Descent 

 of Man, p. 375). It is, of course, employed to 

 express sexual emotion, but is obviously of great 

 use to the bird long before any such emotion is 

 perceived. 



Knapp observed that " birds of one species sing 

 in general like each other, but with different degrees 

 of execution " ; and he recorded a diversity in the 

 songs of the thrush (Journ. Nat., p. 274). Bechstein 

 states that no two species have a similar song 

 (op. cit. p. 2) ; and it is correctly stated that indi- 

 viduals of one species differ widely in their accidental 

 acquirements (IDom. Hab. Birds, p. 291). Rennie 

 observed local variations in bird -song. Thus he 

 remarked : " We have ourselves, in many instances, 

 observed what might not inappropriately be called 

 a different dialect among the same species of song- 

 birds in different countries, and even in places a few 



