The black guillemot or tystie (Cepphus grylle) is a medium-sized seabird of the Alcidae family, native throughout northern Atlantic coasts and eastern North American coasts. It is resident in much of its range, but large populations from the high arctic migrate southwards in winter. The bird can be seen in and around its breeding habitat of rocky shores, cliffs and islands in single or smalls groups of pairs. They feed mainly by diving towards the sea floor feeding on fish, crustaceans or other benthic invertebrates. They are listed on the IUCN red list as a species of least concern.
Both sexes have very similar appearances with black plumage and a large white patch on the upper side of their wings in summer. The bill is also black, being rather long and slender, while the feet are coral-red. In winter adult underparts are white and the upperparts are a pale grey with the back and shoulders exhibiting barred light grey and white patterning.
The birds breed in solitary pairs or small groups during their breeding season starting in late February and early May. Pairs nest above the high tide mark, usually in cliff crevices or gaps in boulders. Many pairs lack true nests and lay directly on the ground. Breeding pairs will typically lay 2-egg clutches and raise 2 chicks to fledging. The eggs are dull white to pale green, boldly marked with dark spots and blotches. Linnaeus specified the type locality as the Ocean of northern Europe but this has subsequently been restricted to Sweden. The black guillemot is now one of three species placed in the genus Cepphus that was introduced in 1769 by the Prussian naturalist Peter Pallas. The genus name Cepphus is from Ancient Greek kepphos, a pale waterbird mentioned by Greek authors including Aristotle. The species name grylle was the local dialect name for this bird in Gotland at the time of Linnaeus's visit there in 1741. The English word "guillemot" is from French guillemot probably derived from Guillaume, "William", but ultimately onomatopoeic from the loud, high-pitched "will, willem" begging calls of the newly fledged young of the common guillemot.
Five subspecies are recognised: They use their wings to propel themselves forward and their feet to steer. One large colony on an island in the Baltic Sea (approximately 2600 individuals) was wiped out by American Mink in the early 2000s.