Guillemot, seabird of the auk family (genera Uria and Cepphus). The black guillemot (C. grylle) is found on Atlantic coasts and the pigeon guillemot (C. columba) on Pacific coasts. Both are black with large white patches on the wings. In winter both become mainly white. Like other auks, guillemots swim underwater with their wings instead of paddling with their feet. They spend most of their time at sea, feeding on fish, and come to land only to breed. They nest on cliffs, usually laying a single egg in rocky clefts.