VOCAL MUSIC. 



367 



sters." Hence bright colors and the power of song seem to re- 

 place each other. We can perceive that if the plumage did not 

 vary in brightness, or if bright colors were dangerous to the spe- 

 cies, other means would be employed to charm the females; and 

 melody of voice offers one such means. 



In some birds the vocal organs differ greatly in the two sexes. 

 In the Tetrao cupido (fig. 39) the male has two bare, orange- 

 colored sacks, one on each side of the neck; and these are largely 

 inflated when the male, during the breeding-season, makes his 

 curious hollow sound, audible at a great distance. Audubon 



*» See remarks to this effect in Gould's 'Introduction to the Trochl- 

 lidae," 1861, p. 2;i. 



