138 CALIFORNIA ACADEMY OP SCIENCES. 



classes are due to the reflection and interference of light 

 waves, and probably all occur to a greater or less extent 

 among birds, although authorities disagree as to which 

 form is the commonest. The iridescent colors are due 

 to these structural peculiarities. White is due to the 

 presence of minute bubbles of gas contained in the 

 feather which breaks the structure so that the light is 

 refracted in all directions, much on the principle of 

 pulverized glass. The worn, dull plumage of birds, at 

 the close of the breeding season, is due to a similar 

 though coarser form of this same agency. Frequently 

 the outer edge of the wing feathers becomes dull brown- 

 ish, or even nearly white, from becoming worn and 

 abraded, even though its natural color be black.* 



The vast majority of color effects, particularly among 

 birds, are due to chemical pigments. Many of these 

 pigments have been chemically analyzed and named, 

 and it is found that the same color in different species 

 is not always due to the presence of one pigment, but 

 to different ones. " Thus," says Beddardf " the brown 

 colour of birds is chiefly due not to one pigment, but to 

 two apparently distinct pigments, which give different 

 chemical reactions; to these two pigments their discov- 

 erer, Kriikenberg, has given the name Zoorubin and 

 Pseudozoorubin. An inspection of the feathers would 

 not enable one to tell with certainty which of the two 

 substances was the cause of the colour; but an extract of 

 zoorubin can always be detected by its change to a beau- 

 tiful cherry red on the addition of the minutest trace of 

 blue sulphate of copper." 



The theory of bird colors which I shall attempt to 

 elaborate in the succeeding pages is as follows: Pig- 



* For fuller discussion of structural colors, see Dr. Gadow, Proc. Zool. 

 Soc., 1882, p. 409; Poulton, Colors of Animals, pp. 3-11. 

 t Animal Coloration, pp. 2-3. 



