EVOLUTION OF THE COLORS OF BIRDS. 315 



or less white or suffused with rusty more or less in- 

 tensely. 



The different characteristic marks were probably de- 

 veloped as discriminative characters first and afterwards 

 modified by sexual selection. 



GENUS TACHYCINETA. WHITE-BELLIED SWALLOW, ETC. 



(8) Adult male more conspicuously colored than 

 female; young with a peculiar first plumage, although 

 similar to adults, the colors being duller. 



Prevailing colors, steel blue, bronze green, purplish, 

 violet, brown, white. 



The beautiful colors of the adults of this genus are 

 the result of sexual selection, the female, particularly 

 of the exquisitely marked violet-green swallow (T. thalan- 

 sina), being much duller than the male. This is an in- 

 stance where Mr. Wallace's theory of the dull colors of 

 female birds as the result of the supression of bright 

 tints from need of protection appears to be utterly un- 

 tenable. White, in contrast to some dark color is the 

 most conspicuous marking in nature. The under parts 

 of the female of this bird are pure white as in the male, 

 but the violet-green colors of the male which are beauti- 

 ful but in no way glaring or conspicuous, are much 

 dulled in the female, conclusively showing that the need 

 for protection is not the factor in this instance. 



GENUS CLIVICOLA. THE BANK SWALLOW. 



GENUS STELGIDOPTERYX. THE BOUGH-WINGED SWAL- 

 LOW. 



(2) Adult male like female; young similar to adult, 

 but some of the feathers with lighter edgings. 



Prevailing colors, brown, gray, white. 



These are the two least specialized species in the group, 

 being not unlike the young of the preceding genus. 



