EVOLUTION OF THE COLORS OF BIRDS. 211 



VARIATION OF COLOR WITH SEX, AGE AND SEASON. 



Before natural selection can be established as a factor 

 in the differentiation of color-patterns, it must be shown 

 that the markings are not perfectly constant. Probably 

 nowhere in the system of the bird is there such scope 

 for individual variation as in pigment distribution. In 

 the first place, no two feathers are ever marked exactly 

 alike. On comparing the pattern even on the right and 

 left wing of the same bird very marked individual diff- 

 erences are at once apparent, although the general effect 

 may be the same. Besides, these individual feather 

 differences which are universally present but which do 

 not alter the general appearance, variations of a more 

 striking nature are not uncommon. The pattern may 

 be more or less sharply defined or intensely colored 

 even in birds of the same age, sex and locality. As a 

 good example of variation in the markings of the tail, I 

 have figured the three outer tail-feathers of the downy 

 woodpecker (Dryobates imbescens) in very pale, normal 

 and in extremely darkly barred examples (Plate I, figs. 

 1-3). Again, the general color may be paler or darker, 

 differences of this sort being very striking in some 

 species, as the meadow lark (Sturnella 'magna), for ex- 

 ample; or, on the other hand, a species may be so con- 

 stant that the closest scrutiny fails to show any well 

 marked individual differences. 



I have by no means attempted in the above survey of 

 recognition marks, to give all the instances under each 

 class, for nearly all birds have them to a greater or less ex- 

 tent, but merely to illustrate and establish the validity 

 of the different forms of recognition markings included 

 in the classification given. A consideration of (C) 

 Epigamic Colors opens up an entirely different field. 

 These are the colors displayed in courtship, and if the 

 conclusions arrived at with regard to sexual selection 



