EVOLUTION OF THE COLORS OF BIRDS. 229 



became red, while the head remained white. It is there- 

 fore quite credible that in a state of nature partial alter- 

 ation of colour may be produced by a change- of diet." 



It is difficult to say whether the change in color of 

 the caged housefinch (Carpodacus mexicanus frontalis) 

 from red to yellow is due principally to a change in food, 

 or to the confinement and general deterioration of the 

 system from captivity. Food, nevertheless, plays some 

 part in this, as well as in many changes in the color of 

 birds in a wild state, which, with the present lack of 

 experimental data, are far too complex even to be sur- 

 mised. 



With regard to the influence of temperature, moisture 

 and sunlight, however, it is less difficult to arrive at 

 some general conclusions. Mr. Frank M. Chapman, I 

 believe, first directed my attention to one of the most 

 obvious and immediate influences of the environment 

 upon color. In certain species a mark is left upon each 

 primary indicating where the superimposed feather cov- 

 ered it. In the grackles (Quiscalus) each primary is a 

 dark brown except the tip which is not covered by the 

 overlying feather, this part being glossy black. So 

 exactly is the boundary defined that the rounded edge 

 of the overlying feather is clearly marked. There can 

 be no possible doubt that this marking is due to the 

 direct influence of the environment, and it seems not 

 improbable that sunlight has been the agent. An an- 

 alogous, though in a way quite different, effect is shown 

 on the wing feathers of the blue bird (Sialia). Here also 

 each primary leaves its impression upon the next feather, 

 but the concealed part, instead of being duller, is brighter 

 blue, the exposed tip being dull brownish. 



It is a very important, and, at the same time, a very 

 difficult matter to determine just what is the effect of 

 sunlight upon pigmentation. By some it has been 



