XII 
THE COLOURS OF BIRDS 
247 
yellow, while in the females of both species they are 
a pale yellow. The brilliant red of the scarlet 
tanager (Pyranga erythromela ) is replaced in the 
western tanager to a large extent by yellow, while 
the females and young of both are yellow. So also 
in the American redstart (Setophaga ruticilla ) the areas 
which in the female are yellow are orange-red in 
the male. In none of these cases do the lipochromes 
appear to have been investigated ; it is most probable 
that in some at least the difference in colour is due 
merely to the amount of pigment present in the 
coloured parts, or it may be in part due to structural 
differences. 
The question as to whether it is possible to 
speak of a geographical distribution of colour is an 
interesting one, but one which it does not appear as 
yet possible to decide. That tropical birds tend 
to be brilliant, and Arctic birds white, appear to be 
as yet almost the only certainties on the subject. 
White as a general ground colour is of considerable 
interest in birds; it is certainly most common among 
the birds of cold climates, but is there often very 
slowly acquired; the gannet, for example, takes 
several years to acquire the pure white adult plum¬ 
age, and so with many others in which the adults 
are pure white. The existence of pure white birds 
is complicated by the frequent occurrence of albinos 
among many species normally coloured ; “ white ” 
blackbirds are, for example, of common occurrence. 
This albinism may be complete, affecting even the 
eye, or it may be confined to special feathers or 
regions of the body. Now the natural whiteness of 
many birds is often compared to these cases of 
