evolution of the colors of birds. 305 



Genus Zonotrichia. The White-crowned Spar- 

 rows, ETC. 



(2) Adult male like female; young like some ances- 

 tral stage of the adult. 



Prevailing colors, black, white, brown, gray, yellow 



This genus has become modified from the primitive 

 bird by losing the streaks on the breast, a corresponding 

 darkening of the back, and by the acquisition of highly 

 characteristic recognition marks about the head. These 

 marks were probably primarily of use as discriminative 

 characters. They would then be most necessary during 

 the breeding season, at which time they reach their 

 maximum development. Without the introduction of 

 any new colors, such striking differences are produced 

 as between Z. querula, with its black head and throat, Z. 

 albicoUis, with its pure white sharply defined patch of 

 white on the throat, and Z. leucophrys, with the character- 

 istic black lines on the head. Still different and equally 

 well marked is Z. coronata in breeding plumage, although 

 in winter these last two birds are not so readily distin- 

 guished at a distance. It is a noteworthy fact that Z. 

 albicoUis, although so differently marked, has the same 

 shade of yellow on the head as Z. coronata, but located 

 on the superciliary stripe, instead of on the median line, 

 as in the latter. This is a fact of some significance with 

 regard to the law of the assortment of pigments. It is 

 also worthy of note that the yellow of Z. albicoUis is dis- 

 tributed as. in Ammodramus, for this may indicate either 

 a genetic relationship or a common cause at work. 



Genus Spizella. The Tree Sparrows, Chipping 

 Sparrows, etc. 



(2) Adult male like female; young like some ances- 

 tral stage of the adult. 



Prevailing colors, black, white, brown, chestnut, gray. 

 2U 



