xii THE COLOURS OF BIRDS 245 



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- 



