XII 
THE COLOURS OF BIRDS 
2 45 
ing case is that of the genus Pericrocotus, in which, 
according to Professor Newton, the males are gener¬ 
ally black and rose-colour and the females gray and 
saffron; this is probably again due to increased 
amount of pigment in the male. So far the sexual 
differences we have noted have depended on the 
development of structural colour in the male, on the 
development of new pigment, or on the increased 
amount of existing pigments. There is, however, 
another difference often marked, and that is the 
relatively greater purity of tint in the male, and the 
frequent presence of contrasting colours. Thus in 
the male blackbird the glossy plumage contrasts 
sharply with the bright yellow beak. In Sericulus 
melinus, one of the regent birds of Australia, the 
female is dull grayish-brown and speckled, while the 
male is black, with brilliant patches of bright orange. 
In the beautiful orioles ( Icterus ) of North America 
the females are olive-green, the males black and 
yellow ; the true orioles of the genus Oriolus show 
the same sexual difference even more distinctly. In 
the North American jays the colours of the males 
are frequently blue, white, and black, with bars and 
spots, while the females and some unspecialised 
species are gray. Facts of this kind are of very 
common occurrence, and have been much insisted 
upon by Mr. Charles Keeler, who regards them 
as tending to prove that the general ground colour 
of the females or of unspecialised species is due 
to a mixture of pigments, while the separation of 
the pigments gives rise to the pure colours of the 
specialised males. The force of Mr. Keeler’s argu¬ 
ments is diminished by the lack of precise dis- 
