EVOLUTION OF THE COLORS OP BIRDS. 291 



and indistinctly streaked indicates the style of coloring 

 of the bird from which this genus, in common with 

 Agelaius and others, was derived. The black color of 

 the male is to be regarded as an indication of specializa- 

 tion, accelerated by sexual selection, utility as a recog- 

 nition mark, and by the principle of sexual intensifica- 

 tion. The only explanation which occurs to me of the 

 brown color of the head and neck of the male is that 

 the specialization has progressed, in accordance with 

 Elmer's law, from posterior to anterior. The bronzed 

 cowbird (M. ceneas), would then be a more specialized 

 form, the black having finally spread over the entire 

 body. The metallic colors are to be regarded as the re- 

 sult of sexual selection. 



Genus Xanthocephalus. Yellow-headed Black- 

 bird. 



(6) Plumage of male in summer different from win- 

 ter plumage; female different from either; young similar 

 to female. 



Prevailing colors, black, white, yellow, brown. 



The yellow color of the head of the male can hardly 

 be explained except as the result of sexual selection, in- 

 completely transferred to the female by inheritance. 

 The white bars on the wing-coverts are undoubtedly 

 recognition marks, and the general black color the re- 

 sult of pigment intensification. That the ancestral bird 

 had a white breast (probably streaked like Molothrus), is 

 indicated by the white mottlings on the breast of the 

 female. There is a marked similarity between the 

 colors, and even the distribution of colors in this genus 

 and in the Arizona hooded oriole (Icterus cucullatus 

 nelsoni), which may be a mere coincidence, but may well 

 have some significance as pointing to an affinity in 

 origin. In both the head is yellow, the back black and 



