Blue and white

Snow geese and blue goose (which is also a snow goose) on a pond near Alda, Nebraska, in the central Platte River Valley. These are lesser snows, Anser caerulescens caerulescens. Millions of lessers migrate through here every spring; we took these photos in large part because of the novelty of seeing fewer than several thousand in one place.

The "blue" form was once considered to be a separate species; it is rare in the greater snow goose (A. c. atlanticus) and extremely rare in the closely related Ross's goose (A. rossii).

Even among lesser snow geese, blues are less common than white snows despite blue being genetically dominant to white. (A similar phenomenon is seen in Australia's Gouldian finch: red-headed Gouldians outnumber black-headed ones by a wide margin, but black is genetically dominant to red.) 

Geese, as a group, imprint hard (think of Konrad Lorenz and his greylags), and snow geese preferentially mate with geese resembling their parents. A mixed pair such as this one (I took them to be a pair, at any rate) will likely have a mixed clutch of blues and snows, who as adults will pair with either blues or snows themselves.