80 
sexual selection: bikds. 
PahT* 
the males when adult are either retained for life or 
periodically renewed during the summer and breedi"? 
season. At this season the beak and naked skin ab° 
the head frequently change colour, as with some hero 1 '" 
ibises, gulls, one of the bell-birds just noticed, &c. 
the white ibis, the cheeks, the inflatable skin of 1 
throat, and the basal portion of the beak, then becO^ 
crimson . 71 In one of the rails, Gatticrex cristatus a h"-'^ 
red caruncle is developed during this same period 
the head of the male. So it is with a thin horny C 1 , 
on the beak of one of the pelicans, P. erythrorhynch l>s ' 
for after the breeding-season, these horny crests ^ 
shed, like horns from the heads of stags, and the si)'’ 1 , 
of an island in a lake in Nevada was found cove r 
with these curious exuvke . 72 
dU 
Changes of colour in the plumage according to 
season depend firstly on a double annual moult, secoo 1 "; 
on an actual change of colour in the feathers themself 
!<* 
and thirdly on their dull-coloured margius being per 1 ' 
ically shed, or on these three processes more or 
combined. The shedding of the deciduary margins 
be compared with the shedding by very young bA 1 
of their down ; for the down in most cases arises f rC * I 
the summits of the first true feathers . 73 
With respect to the birds which annually undergo 
double moult, there are, firstly, some kinds, for inst#",. 
snipes, swallow-plovers (Glared®), and curlews, ^ 
which the two sexes resemble each other and do 11 . 
change colour at any season. I do not know whet^, 
the winter-plumage is thicker and warmer than 11 
U 1 Land and Water,’ 1867, p. 394. 
w Mr. D. G. Elliot,, in ‘Proo. Zool. Soc.’ 1869, p. 589. ^ 
7 3 ‘Nitzsck’s Pterylograpliy,’ edited by P. L. Selater. Bay ^ 
18G7, p. 14. 
