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Guillemot is the common name for several species of seabird in the Alcidae or auk family (part of the order Charadriiformes). In British use, the term comprises two genera: Uria and Cepphus. In North America the Uria species are called "murres" and only the Cepphus species are called "guillemots". This word of French origin derives from a form of the name William, cf. French: Guillaume.
The two living species of Uria, together with the razorbill, dovekie and the extinct great auk, make up the tribe Alcini. They have distinctly white bellies, thicker and longer bills than Cepphus, and form very dense colonies on cliffs during the reproductive season. Guillemot eggs are large (around 11% of female weight), pointed at one end, and water-repellent and self-cleaning.
The three living species of Cepphus form a tribe of their own: Cepphini. They are smaller than the Uria species and have black bellies, rounder heads and bright red feet.
- Common murre or common guillemot, Uria aalge
- Thick-billed murre or Brünnich's guillemot, Uria lomvia
Some prehistoric species are also known:
- Uria bordkorbi (Monterey or Sisquoc Late Miocene of Lompoc, USA)
- Uria affinis (Late Pleistocene of E USA)—possibly a subspecies of U. lomvia
- Uria paleohesperis
U. brodkorbi is the only known occurrence of the Alcini tribe in the temperate to subtropical Pacific, except for the very fringe of the range of U. aalge.
- Black guillemot or tystie, Cepphus grylle
- Pigeon guillemot, Cepphus columba
- Spectacled guillemot, Cepphus carbo
As in other genera of auks, fossils of prehistoric forms of Cepphus have been found:
- Cepphus olsoni (San Luis Rey River Late Miocene—Early Pliocene of W USA)
- Cepphus cf. columba (Lawrence Canyon Early Pliocene of W USA)
- Cepphus cf. grylle (San Diego Late Pliocene, W USA)
The latter two resemble the extant species, but because of the considerable distance in time or space from their current occurrence, they may represent distinct species.