BIRDS 115 



native haunts in Asia congregate in small flocks, but very- 

 early in spring separate into pairs, when the males begin to 

 crow. The fact that they are monogamous explains why they 

 do not display and have not developed sexual dimorphism. 



In Numida, the guinea-fowls, placed in the same family, 

 there is no difference between the sexes. The cocks do not 

 fight and are monogamous. 



In the sub-family Tetraonime there are species in which 

 sexual dimorphism is strongly developed, and others in which 

 there is little or no difference between male and female. 

 Darwin has pointed out that this contrast corresponds to the 

 polygamy of some species and the monogamy of others, but it 

 is the differentiation of habit associated with polygamy which 

 really affords the explanation of the matter, 



The capercailzie and the black-cock {Tetrao uragallus and 

 T. tetrix) are both polygamists, and have regular places where 

 they meet together to fight and display their plumage before 

 the females. Their principal weapons are their beaks. The 

 capercailzie differs from the female in size; in the black- 

 cock the colour is black, while that of the hen is gray, and 

 the outer feathers of the cock's tail are elongated and curved 

 outwards. 



Darwin has described the unisexual peculiarities of the 

 male Tetrao cupido, or prairie cock of North America, and 

 has figured the bird in the attitude assumed during courtship. 

 There is a bare orange-coloured sac on each side of the neck, 

 which is inflated when the male utters his loud note, and 

 which is obviously to be compared with the sacs in the 

 throat of the howling monkey, producing a similar effect, and 

 due to similar causes. There are two ear-tufts of feathers, 

 considerably elongated, and these are erected so as to meet 

 over the head. 



Tetrao itrophasianus of North America affords similar 

 illustrations of the precise correspondence between structure 



