244 COLOUR IN NATURE chap. 



on the beak, or the skin of the head, as in some 

 birds of paradise, is always structural. The naked 

 patches of skin in birds indeed exhibit the same 

 tendencies with regard to colouring as are visible in 

 the feathers. 



Sexual Coloration 



Before giving a detailed account of the colours 

 and colouring-matters of birds we may summarise 

 some of the general characters of the coloration. 

 One of the most marked characters of the group is 

 the great prevalence of sexual differences in colour, 

 usually though not invariably of such nature that 

 the male excels the female in brilliancy, or at least 

 in intensity of colour. The colour differences are of 

 many kinds. Thus, as in the humming-birds and 

 the peacock, the male may display brilliant struc- 

 tural colour, absent or feebly developed in the 

 female ; or as in some of the birds of paradise, the 

 plumage of the male may be coloured by special 

 pigments which are absent in the female. Again, 

 as in our own blackbird, the male may, as compared 

 with the female, merely exhibit a greater intensity 

 of colour. The same fact is illustrated in a curious 

 way in the case of the satin bower bird {Ptilono- 

 rhynchus violaceus) ; here the female has a grayish 

 somewhat thrush-like plumage exhibiting through- 

 out faint but distinct metallic tints, so that the 

 whole bird has a delicately iridescent appearance. 

 The male, on the other hand, is a deep glossy black, 

 with a hard metallic lustre — a more specialised, if 

 rather less beautiful colour. Another very interest- 



