EVOLUTION OF THE COLORS OF BIRDS. 295 



especially well illustrated in the /. cucullatus group 

 which becomes intensified from a yellow at its northern 

 limit to an intense flame in the tropics. In /. bullocid 

 the black has extended over the head, leaving a super- 

 ciliary stripe, however. In I. galhula the black has 

 spread over the sides of the head connecting the throat 

 patch and back. In /. parisorum and I. spiirius it has 

 extended still farther down on the breast. There is 

 apparently here a tendency towards the assumption of a 

 completely black plumage as the bird becomes more and 

 more specialized. The reddish brown color of /. spurius 

 seems very different from the rest of the group, but I 

 do not think it has been produced by the introduction 

 of a new pigment but rather by a modification of the 

 yellow. It is apparent that the color of the ancestral 

 form is yellow, from the persistence of this color in the 

 young. By intensification this yellow becomes red as 

 in the analogous case of /. cucullatus, and it is perfectly 

 conceivable that by further intensification, even by the 

 simple increase in the amount of the pigment, the red 

 would become darker instead of more brilliant. 



Genus Scolecophagus j The Blackbirds and 



Genus Quiscalus ' Grackles. 



(7) Adult male more conspicuously colored than 

 female; young similar to adult female. In some species 

 a seasonal difference (6) in which the male differs from 

 the female in both summer and winter plumage. 



Prevailing colors, black, brownish, various metallic 

 hues. 



General pigment intensification, sexual selection, and 

 the advantage of conspicuous colors for gregarious birds 

 serve to explain the colors of these two genera. 



