270 CALIFORNIA ACADEMY OF SCIENCES. 



these two species being exceptional among hawks for 

 their susceptibility to environmental changes. 



Genus Urubitinga. Mexican Black Hawk, etc. 



(2) Adult male like female; young with a peculiar 

 first plumage. 



Prevailing colors (adult) black, white; (young) brown, 

 ochraceous. 



The adults in this genus have completely attained the 

 black plumage with the exception of the white recogni- 

 tion marks of the tail. The plumage of the young 

 probably has no reference to utility but is simply at a 

 less advanced stage of pigmentation. The different 

 species are distinguished by the distribution of white 

 marks on the tail, which afford an excellent illustration 

 of discriminative marks. 



Genus Asturina. Mexican Goshawks. 



(2) Adult male like female; young with peculiar first 

 plumage. 



The remarks made concerning the genus Accipiter 

 would apply equally to the present bird, the general 

 features of coloration being about the same. 



Genera Aquila, Thras^eetus, Halleetus. The 

 Eagles. 



(2) Adult male like female; young with peculiar first 

 plumage. 



Prevailing colors, black, white, gray, brown. 



The markings of the eagles, perhaps, more than of 

 any other land birds, are to be explained almost exclu- 

 sive by the general laws of growth. Very possibly such 

 marks as the white thighs with black bars in Thrasasetus, 

 the white tail of Halia^etus, and the white head of H. 

 leucocephalus , were developed as discriminative, or pos- 

 sibly to some extent as socialistic, marks. The plum- 



