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 {Pyrdnga erythromeld) 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 ruticilld) 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 



