Ross's Goose, Chen Rossii
Ross's Geese are similar to Snow Geese, and in most places, much less common. However at some wildlife refuges in the California Central Valley, notably Merced and Colusa, Ross's outnumber Snow. Ross's are smaller, with a proportionately smaller bill, and a more rounded head, with an angle between forehead and bill, where on a Snow Goose there would be a triangular wedge of head and bill together. Snow Geese also show a prominent dark line -- the "grinning patch" -- between the mandibles of the bill, while Ross's Geese usually have a gray area at the base of their pink bills. Unlike Ross's Geese, Snow Geese often have their heads stained orange from iron in the mud they feed in. These differences are illustrated below, in a photograph showing birds of both species together.
This picture shows a mixture of Snow and Ross's Geese; when they are seen side by side, the features that distinguish them are readily noticed. All six of the birds still in flight are Snows; most of the birds already in the water are Ross's, with a few Snows. A larger version of this picture, allowing closer examination of the differences between birds of the two species, can be seen here.
One of the great sights at California Central Valley refuges is the "blast-off" of thousands of geese at once. Here's a glimpse, a fraction of a flock of Ross's Geese, but no photograph can do it justice.