guillemot (gĬl´əmŏt´), northern sea bird, genus Cephas, of the auk family. The black guillemot, or trystie, Cephus grylle, is about 13 in. (33 cm) long and is very striking in its breeding plumage, being entirely black from bill to tail except for large white wing patches and bright red legs. In winter its plumage is a mixture of black, white, and gray. It inhabits coasts all around the North Atlantic Ocean, but is more abundant on the American side, where it is found as far south as Cape Cod. The pigeon guillemot, C. columba, is very similar to the black guillemot, and may be a geographic variety rather than a distinct species. It is found on Pacific coasts from the Bering Sea to S California and N Japan. Guillemots are distinctive among auks in a number of ways: they are better fliers than most, they are not very gregarious, and the female lays two or three eggs at a time instead of the usual one. Pairs of guillemots tend to occupy particular territories in the water, where they swim and dive for food. Like other auks, they build no nest, but lay their eggs on the rocks. In Great Britain a related bird, called murre in North America, is also called guillemot. Guillemots are classified in the phylum Chordata, subphylum Vertebrata, class Aves, order Charadriiformes, family Alcidae.