CHAPTEK II 



BIRDS 



Differences between male and female are more conspicuous 

 in birds than in any other class of animals, chiefly in con- 

 sequence of the great development and brilliant coloration 

 which parts of the plumage exhibit. The sexual dimorphism 

 reaches its maximum in three families : the Trochilidae or 

 humming-birds, the Paradiseidse or birds of Paradise, and the 

 Phasianidae or game-birds. 



In the mammals, as we have seen, unisexual male 

 characters when present belong usually to the first of the 

 kinds which I have distinguished in the Introduction ; they 

 are either weapons or modifications produced as the result of 

 blows and impacts habitually inflicted in the fights of rival 

 males. In birds characters of the second kind are more 

 common, that is characters or appendages which are em- 

 ployed in courtship to please or excite the female. Characters 

 of the third kind, those which are due to the effect of 

 different modes of life in the two sexes, not related to 

 warfare or courtship, also occur, but are less common. 



With regard to the plumage, the chief point which I wish 

 to emphasise is the invariable association between excessive 

 development and erection or display of the feathers. Darwin 

 has collected and set forth the facts bearing upon the gestures 



