so 
SEXUAL selection: bikes. 
Part II. 
tlie 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 just 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, OalUcrex 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. eryihrorliynclius ; 
for after the breeding-season, these horny 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 change 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 annually undergo a 
double moult, there are, firstly, some kinds, for instance 
snipes, swallow-plovers (Glareolsc), 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 
^ Land and Water/ 1867, p. 394. 
Mr. D. G. Elliot, in ‘Proc. Zool. Soc.’ 1869, p. 589. 
‘ Nitzsch’s Pterylography,’ edited by P. L. Sclater. Kay Soc. 
1867, p, 14. 
