SEXUAL SELECTION- 483 



if bright colors were dangerous to the species, other means 

 would be employed to charm th« 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 sacs, one on each side of the neck; and 

 these are largely inflated when the male, during the breed- 

 ing season, makes his curious hollow sound, audible at a 

 great distance. Audubon proved that the sound was in- 

 timately connected with this apparatus (which reminds us 

 of the air-sacs on each side of the mouth of certain male 

 frogs), for he found that the sound was much diminished 

 when one of the sacs of a tame bird was pricked, and 

 when both were pricked it was altogether stopped. The 

 female has "a somewhat similar, though smaller, naked 

 space of skin on the neck; but this is not capable of in- 

 flation."" The male of. another kind of grouse {Tetrao 

 urophasianus), while courting the female, has his "bare 

 yellow oesophagus inflated to a prodigious size, fully half 

 as large as the body' ' ; and he then utters various grating, 

 deep, hollow tones. With his neck-feathers erect, his wings 

 lowered, and buzzing on the ground, and his long pointed 

 tail spread out like a fan, he displays a variety of grotesque 

 attitudes. The oesophagus of the female is not in any way 

 remarkable." 



It seems now well made out that the great throat-pouch 

 of the European male bustard {Otis tarda), and of at least 

 four other species, does not, as was formerly supposed, 

 serve to hold water, but is connected with the utterance 

 during the breeding season of a peculiar sound resembling 

 "ock."*' A crow-like bird inhabiting South America 



« "The Sportsman and Naturalist in Canada," by Major W. Eoss King, 

 1866, pp. 144-146. Mr. T. W. Wood gives in the "Student" (April, 1810. p. 

 116) an excellent account ol the attitude and habits of this bird during its 

 courtship. He states that the ear-tufts or neck-plumes are erected, so that 

 they meet over the crown of the head. See his drawing, Kg. 39. 



•** Richardson, "Fauna Bor. American: Birds," 1831, p. 359. Audubon, 

 SUd., vol. Iv. p. 60T. 



« The following papers have been lately written on this subject: Prot A. 



