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 familiar 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 
