80 SEXUAL SELECTION : BIEDS. Part IL 



the males when adult are either retained for life or are 

 periodically renewed during the summer and breeding- 

 season. At this season the beak and naked skin about 

 the head frequently change colour, as with some herons, 

 ibises, gulls, one of the bell-birds jnst noticed, &c. In 

 the white ibis, the cheeks, the inflatable skin of the 

 throat, and the basal portion of the beak, then become 

 crimson.'^ In one of the rails, GalUcrex cristatus a large 

 red caruncle is developed during this same period on 

 the head of the male. So it is with a thin horny crest 

 on the beak of one of the pelicans, P. erijthrorhynGhus ; 

 for after the breed in o-season, these liornv crests are 

 shed, like horns from the heads of stags, and the shore 

 of an island in a lake in Nevada was found covered 

 with these curious exuvise.^^ 



Changes of colour in the plumage according to the 

 season depend firstly on a double annual moult, secondly 

 on an actual chancre of colour in the feathers themselves, 

 and thirdly on their dull-coloured margins being period- 

 ically shed, or on these three processes more or less 

 combined. The shedding of the deciduary margins may 

 be compared with the shedding by very young birds 

 of their down ; for the down in most cases arises from 

 the summits of the first true feathers.'^^ 



With respect to the birds which aimually undergo a 

 double moult, there are, firstly, some kinds, for instance 

 snipes, swallow-plovers (Glareolae), and curlews, in 

 which the two sexes resemble each other and do not 

 change colour at any season. I do not know whether 

 the winter-plumage is thicker and warmer than the 



71 ' Land and Water,' 18G7, p. 394. 

 "-' Mr. D. G. Elliot, i:i 'Proe. ZooL Soc' 1803, p. 589. 

 '3 ' Nitzsch's Pterylogi aphy,' edited by P. L. Sclater. Kay Soc. 

 1867, p. 14. 



