218 SEXUAL SELECTION: BIRDS. [Part II. 



other birds should be encumbered with plumes so long as 

 to impede their flight. 



In the same manner, as the males alone of various spe- 

 cies are black, the females being dull-colored ; so in a few 

 cases the males alone are either wholly or partially white, 

 as with the several Bell-birds of South America (Chasmo- 

 rhynchus), the Antarctic goose (Bernicla antarctica), the 

 silver-pheasant, etc., while the females are brown or ob- 

 scurely mottled. Therefore, on the same principle as 

 before, it is probable that both sexes of many birds, such 

 as white cockatoos, several egrets with their beautiful 

 plumes, certain ibises, gulls, terns, etc., have acquired 

 their more or less completely white plumage through sex- 

 ual selection. The species which inhabit snowy regions 

 of course come under a different head. The white plu- 

 mage of some of the above-named birds appears in both 

 sexes only when they are mature. This is likewise the 

 case with certain gannets, tropic-birds, etc., and with the 

 snow-goose (Anser hyperboreus). As the latter breeds on 

 the " barren grounds," when not covered with snow, and 

 as it migrates southward during the winter, there is no 

 reason to suppose that its snow-white adult plumage 

 serves as a protection. In the case of the Anastomus 

 oscitans previously alluded to, we have still better evi- 

 dence that the white plumage is a nuptial character, for it 

 is developed only during the summer ; the young in their 

 immature state, and the adults in their winter dress, being 

 gray and black. With many kinds of gulls (Larus), the 

 head and neck become pure white during the summer, 

 being gray or mottled during the winter and' in the young 

 Btate. On the other hand, with the smaller gulls, or sea- 

 mews (Gavia), and with some terns (Sterna), exactly the 

 reverse occurs ; for the heads of the young birds during 

 the first year, and of the adults during the winter, are 

 either pure white, or much paler-colored than during the 



