Breeding adults are mostly deep black with large white patches on the wings visible when the wings are folded. The underwings are white with black border. The legs and feet are bright red. Nonbreeding adults retain the black-and-white wing pattern but have mostly whitish head, neck, and underparts. Immatures are similar but with grayish feathers in the head, neck, and sides, and less white in the upperwing.
Nests on rocky islands, shorelines, and cliffs. Forages in littoral (near-shore) ocean waters and openings in sea ice.
Ornithologists have described up to 7 subspecies, 2 or 3 of which nest in North America. In Maine and southeastern Canada, arcticus is the nesting subspecies, while in Alaska, mandtii is the nesting subspecies. Some ornithologists also recognize ultimus as a distinct subspecies in the High Arctic of eastern Canada and Greenland; others combine it with mandtii. In adult nonbreeding plumage, mandtii and ultimus have snowy white body plumage; juvenile mandtii/ultimus havewhite-tipped secondary feathers, forming a white trailing edge to the wing. Some Black Guillemots that nest in western Greenland are mostly black in breeding plumage7, with little or no white on the wings. Such “dark-morph” birds are also much darker in nonbreeding plumage than others in this population, which acquire mostly snow-white plumage in winter.