EVOLUTION OF THE COLORS OP BIRDS. 289 



to the female and young. The influences of isolation 

 and environment upon the genus Cyanocitta has been 

 already discussed (see ante, p. 245). Isolation and en- 

 vironmental influences have also been responsible for 

 most if not all of the forms of Aphelocoma. The un- 

 usual number of closely allied species or varieties in- 

 habiting the different parts of southern North America 

 is to be accounted for by the fact that the bird is gen- 

 erally resident wherever found, and not much given to 

 wandering, except in a local manner when food is scarce 

 in a particular locality. 



The colors of the magpies (Pica) were probably first 

 developed as recognition marks and afterwards enhanced 

 by sexual selection. The habits of the magpies are such 

 that recognition marks would be of great service, par- 

 ticularly in their social relations. The yellow bill and 

 smaller size of P. nuttalli is doubtless the result of geo- 

 graphical isolation, and not a -character to which any 

 utility can be ascribed. Perisoreus from the somber 

 forests of the north has simply never acquired a special- 

 ized plumage, since the labor of gaining a living must 

 have quite monopolized the bird's attention, and en- 

 tirely driven any sesthetic tendencies out of its head. The 

 different species and varieties are largely the result of 

 climatic influences. Clarke's nutcracker (Picicorvus 

 columbianus) is marked with conspicuous black and 

 white recognition characters on the wings and tail, the 

 rest of the body being of the generalized gray of the 

 family. The pinon jay ( Gyanocephalus cyanoceiolialus) is 

 without decided recognition marks, but has the ground 

 color a pale blue, not unlike that of Aphelocoma. The 

 black of the crows and ravens is simply the completion 

 of color specialization, but it is doubtless also useful as 

 a general recognition mark. There can apparently be 

 no possible utilitarian significance in the white basal 

 19 



