Breeding birds: 19,000 pairs
Wintering birds: 58-80,000 birds
Adult black guillemots in breeding plumage are entirely black or very dark brown except for a white patch on the uppwerwing and white undertail coverts. Some birds may occasionally be missing the wing patches.
They have a thin, pointed black bill with a bright red mouth-lining, dark brown eyes, bright red legs and webbed feet with black claws.
Out of breeding plumage black guillemots are light grey with white wing patches. Their head and underparts are white and their upperparts are speckled with light grey, black, and white.
Juvenile black guillemots are similar to adults in winter. Their upperwing patches are mottled and their wing coverts have brown tips.
Black guillemots breed between late May and mid June. They breed in small scattered colonies. The nest is in a rock crevice at the base of a cliff or in a cavity in the cliff face.
Black guillemots lay 1-2 white eggs with dark markings which are incubated by both adults for 28-32 days. Chicks are semi-precocial and covered in dark down, but are fed by both parents until they fledge at 3-40 days. They reach sexual maturity at 3-4 years.
Black guillemots eat mainly fish but further north will also feed on crustaceans.
Black guillemots can be seen in the UK all year round. They are found down the west coast of Scotland and the North West of England, along the Northern Ireland coast, and on the Isle of Man.
Black guillemots carry fish crosswise in their bills, sometimes for hours at a time, a behaviour described by Edmund Selousin his book The Bird Watcher in the Shetlands published in 1905.