296 The Bird 



forms entirely different, and thought to be due solely 

 to the amount of moisture in the ground on which it lives. 

 Very dark-coloured and very pale individuals live within 

 a few hundred yards of each other, in dry and swampy 

 situations respectively, each, it is said, keeping entirely 

 to its own little beat. 



We are all fairiliar with the changes of colour due to 



FIG. 236. Nighthawk perching lengthwise on a fallen branch. 



age, as, for instance, in the young Rose-breasted Gros- 

 beaks, which are very different from the male parent, and 

 the young Bald Eagles, which lack the white colour of the 

 feathers of head and tail. Certain wild pigeons show 

 marked differences in colour patterns between the young 

 birds and the adults, and very good evidence of the gradual 

 evolution which must have preceded these changes is 



