Adult birds are black on the head, back and wings with white underparts. The thin dark pointed bill. They have a small rounded dark tail. The face becomes white in winter. Western European birds of the race U. a. albionis are dark brown rather than black, most obviously so in colonies in southern Britain.
Their breeding habitat is islands, rocky shores, cliffs and sea stacks on
These birds forage for food by swimming underwater. They mainly eat fish, also some crustaceans, marine worms and squid.