210 THE STORY OF THE BIRDS 



birds that we might almost describe them as of 

 universal prevalence. It may be seen in many 

 different forms, but its most frequent type is 

 that of general colouration. Innumerable in- 

 stances may be called to mind by every reader 

 at all familiar with birds in which the male is 

 more showily dressed than the female, as, for 

 example, in the Blackbird. More striking in- 

 stances are furnished by species in which the 

 male bird is adorned with crests or plumes, or 

 other nuptial ornaments, or is armed with spurs, 

 or decorated with tubercles, wattles, combs, and 

 so forth. Sexual dimorphism also extends to 

 the organs of voice, and such peculiarities as 

 pouches and sacs. We have already noticed 

 sexual dimorphism in the bill of the New Zea- 

 land Huia {conf, p. 194). In not a few cases it 

 is manifest in size. In great numbers of birds 

 the female is considerably bigger than the male, 

 especially in the Birds of Prey ; in other cases 

 she is smaller, notably among the Galliformes 

 and Ratita3 Birds. 



Of dimorphism independent of sex the in- 

 stances are much rarer ; indeed, but few cases 

 are known among birds. One of the most 

 remarkable of these is furnished by several 

 species of Skuas, which have two very distinct 



