The common guillemot is a rather dumpy, thick set auk reaching a maximum size of about 46 cm, and with a wingspan up to 73 cm. The breeding plumage has dark chocolate brown upperparts and head to the lower throat, and white underparts with some dark streaking on the flanks. Between different populations there is variation in the extent of this streaking. During the winter the sides of the face and throat are white. However, the extent of this colouration also varies between populations. The bill is long, slender and pointed, and the feet are black. A 'bridled' form occurs with a white eye ring and white stripes behind the eye. Juveniles resemble adults but display a variable amount of white on the throat.