The Snow Goose is a striking Nearctic-breeding waterbird that winters from Mexico north to the central United States. Staggered waves of these flickering black-and-white geese moving over North America in fall in spring have captivated humans for ages. Snow Geese frequently winter on the northcentral plateau of Mexico as well as on the Gulf Coast, with numbers reaching the shores of Baja California. Polymorphic, the species occurs in two main color "themes" with many intermediate variations possible as well. White morph Snow Geese are all white with black primaries and wing coverts, with a stout pink bill marked by a black tomial "grin patch", and pink legs. Dark morph birds are highly variable, but are typically dark gray-brown on the body and flight feathers, white on the head, with blue-gray wing coverts contrasting with black flight feathers. The seemingly unlimited availability of grain and other food on the wintering grounds has allowed this species to thrive, and fast population expansion threatens some fragile Arctic breeding sites where Snow Geese breed in massive colonies.