It is interesting to note that almost every part of the bird, whose variations can be noted and selected, has led to variations of a considerable extent, and many of these have necessitated changes in the plumage and in the skeleton quite as great as any that occur in the numerous distinct species of large genera.

The form of the skull and beak varies enormously, so that the skulls of the Short-faced tumbler and some of the Carriers differ more than any wild pigeons, even those classed in distinct genera.

The breadth and number of the ribs vary, as well as the processes on them; the number of the vertebrae and the length of the sternum also vary; and the perforations in the sternum vary in size and shape.

The oil gland varies in development, and is sometimes absent.

The number of the wing-feathers varies, and those of the tail to an enormous extent.