The period at which the perfect plumage is acquired varies, as does the state of the down with which the nestling birds are clothed when hatched.
The shape and size of the eggs vary.
The manner of flight, and in some breeds the voice and disposition, differ remarkably.
Lastly, in certain breeds, the males and females have come to differ in a slight degree from each other.
Altogether at least a score of pigeons might be chosen, which, if shown to an ornithologist, and he were told that they were wild birds, would certainly be ranked by him as well-defined species.