Nearly all birds change plumages from one season to another, but there are a few northern species that change from bright summer breeding plumages to white in winter in order to blend with the snow. Among these snowbirds are the ptarmigans, chickenlike members of the grouse family, that change from rusty browns and grays in summer to nearly all-white in winter, so that they can better hide from predators.
The opposite is true with the snowy owl. This bird of prey, which becomes whiter as it matures, uses its whiteness as camouflage to help it capture more prey in winter.
There are two songbirds that are nearly all-white in winter. McKay’s buntings and snow buntings blend so well with a snowy landscape that they can be seen along roadsides in winter only when they fly. In summer, both buntings have some white mixed with black in their feathers to hide them in their far northern breeding grounds. In winter, their white is mixed with reddish feathers, apparently to blend with their farmland wintering grounds farther south.
One species of waterfowl, the long-tailed duck (formerly called oldsquaw), which winters along northern coastlines, turns predominantly white from more brown and white in summer.
There are other all-white birds, such as egrets, ibises and herons, that spend the winter in warm climates, where they may get some protection from a lack of pigment in their feathers, but it isn’t from blending with snow.
-- George H. Harrison