The common guillemot is the largest of the extant auk species. The common guillemot is one of the most abundant seabirds in temperate and colder parts of the northern hemisphere, with very large populations in the Atlantic and the Pacific Oceans, and adjacent areas of the Arctic Ocean. In the northeast Atlantic its range extends from Portugal in the south to Svalbard and Novaya Zemlya in the north and includes the Baltic. Bjørnøya is the most important breeding area for the common guillemot in Svalbard and the entire Barents Sea.
Male and female common guillemots have the same external appearance. Adult birds are 41 cm long and weigh 900–1100 g. They are brown-black on the upper-parts and white on the under-parts. In breeding plumage, the whole head is dark, and the white breast is sharply shaped into a wide-angled peak towards the dark throat.
Some individuals, the so-called ‘bridled’ morph, have a white ring around the eye and a white stripe extending from the eye backwards towards the neck. The frequency of the bridled morph increases from near 0 % at the extreme south of the species’ range to more than 50 % in the northern part of the range in Svalbard.
The common guillemot is distinguishable from the Brünnich’s guillemot by the longer and more pointed bill without the white stripe at the base of the upper mandible, and by the dark mottled marks along the flanks. In winter, the neck and sides of the head are white with a dark stripe from the eye across the ear-coverts.
The juveniles resemble the adults in winter plumage, but have shorter bills.
The calls when at the nesting site sound like a grating ‘arrr’.
The common guillemot is one of the most abundant seabirds in temperate and colder parts of the northern hemisphere, with very large populations in the Atlantic and the Pacific Oceans, and adjacent areas of the Arctic Ocean. In the northeast Atlantic its range extends from Portugal in the south to Svalbard and Novaya Zemlya in the north and includes the Baltic. The common guillemot has a more southerly breeding distribution than the Brünnich’s guillemot, but the two species do overlap in some places.
Six sub-species have been described within the North Atlantic, but they are poorly differentiated. The Svalbard population belongs to the race U. a. hyperborea.
Bjørnøya is the most important breeding area for the common guillemot in Svalbard and the entire Barents Sea. On Bjørnøya, it breeds in large colonies on ledges and plateaus, often together with the Brünnich’s guillemot.
The common guillemot is a dispersive rather than migratory species with a significant fraction of the adult birds remaining within a few hundred kilometres of the colonies throughout the year. The Svalbard birds probably winter in the southern parts of the Barents Sea and in coastal waters off northern Norway. The birds leave the colonies in late July–early August and return in late March–early May.
The common guillemot is extremely gregarious and colonial breeding is the norm. The colonies can contain many tens of thousands of individuals.
Common guillemots nest exclusively in steep cliffs, either on narrow ledges or platforms. In mixed colonies, the common guillemot is usually found on the broadest ledges and plateaus.
Breeding success is highest where birds breed at high density or where sites are protected from predators. The Arctic fox, glaucous gull and great black-backed gull are important predators of eggs, chicks and adult birds.
Outside the breeding season, the common guillemot appears in both inshore waters as well as further out to sea.
The common guillemot is predominantly piscivorous, preferentially consuming small (max. ca. 200 mm long) schooling fish which it catches underwater. The capelin Mallotus villosus is the principal food source for common guillemots on Bjørnøya. The common guillemot can dive to depths of more than 150 metres, but normal feeding depth is probably 20–50 m. The diet suggests that prey is mostly taken in the middle of the water column, rather than being taken from the bottom.
Life history and reproduction
Life history and reproduction: Egg-laying starts at the end of May or early in June. Common guillemots lay their eggs directly on bare rock or soil. The single egg is pear-shaped, preventing it from rolling off the ledge. The egg varies in colour from a light blue-green to brown with dark streaks and blotches.
The egg is incubated by both parents for about 32 days. The young bird leaves the shelf before becoming fully fledged, at about 21 days of age. Departure of the young from the colony is highly synchronised within a few days and most young leave during the night. Where the colonies are not close to the sea, the chicks usually land on the ground below the cliff and have to walk the last stretch to the sea. The chicks are followed at sea by the male parents who feed and take care of them during the period after leaving the nest.
The chick becomes independent 10–12 weeks later. Males and young carry out a swimming migration from the colony out to sea. Following the chick-rearing period the parents moult and are unable to fly for 45–50 days.
Common guillemots normally become sexually mature at five years of age, but immature birds often visit breeding colonies in their third or fourth year.
Management status and monitoring
The breeding population on Bjørnøya was estimated to be 245,000 pairs in 1986. In 1987, the number of breeding pairs was reduced to 36,000 pairs; a reduction of 85 %. This dramatic decline was most likely due to the collapse of the Barents Sea capelin stock, which was caused by oceanographic change, perhaps in concert with over-fishing. Many birds did not breed when food was scarce in the years following the capelin collapse. The number of breeding pairs has increased since 1987, returning to the level of 1986 by 2013.
Only about 100–200 pairs breed in Svalbard outside Bjørnøya. On the Norwegian mainland, the population of common guillemots declined dramatically in the 1960s and 1970s in all colonies west of Nordkapp, and some of the colonies are today considered to be seriously threatened with extinction. The reason for this decline is likely a composite of mortality in fishing gear, hunting and food shortages.