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The common eider is a large diving duck that is easily distinguishable even at quite long distances because of the elongated profile of the head. It has a circumpolar distribution and breeds in the arctic and boreal zones of the northern hemisphere. They nest along the coast of Europe from northern France, through the Netherlands and the British Isles to Iceland and northwards through Scandinavia to the arctic coasts of Russia as well as in arctic regions such as Svalbard, north-eastern Siberia, arctic North America and Greenland.
The common eider is a large diving duck that is easily distinguishable, even at quite long distances, because of the elongated profile of the head. Adults of this species are approximately 58 cm long and weigh from 1.2–2.8 kg.
Males and females have very different plumage; the male has a black crown, belly and tail, while the rest of the plumage is white, the breast usually has a pinkish tinge. Males also have characteristic green marks on the sides of the head. The female is brown with light, closely packed bars over the whole body.
Young males are a mottled brown-black and white, the pattern of which varies with age. Young birds are difficult to distinguish from adult males in transitional plumage after the nesting season, when they are switching to their winter colours.
The male’s voice is a deep, prolonged ‘coo-roo-uh’, the female’s call is a growling ‘cor-r-r’.
The common eider has a circumpolar distribution and breeds in the arctic and boreal zones of the northern hemisphere. They nest along the coast of Europe from northern France, through the Netherlands and the British Isles to Iceland and northwards through Scandinavia to the arctic coasts of Russia as well as in arctic regions such as Svalbard, north-east Siberia, the arctic North America and Greenland. They winter largely within the breeding range, migrating southward from only the most northerly regions.
Several sub-species have been described. The Svalbard population belongs to the sub-species Somateria mollissima borealis, which also breeds in northeast Canada, Greenland, Iceland and Franz Josef Land. In Svalbard the common eider breeds both in colonies on small islands and in a more dispersed fashion along the coast of Spitsbergen. The greatest nesting densities occur on the western and north-western coast of Spitsbergen and on Tusenøyane. The Svalbard population winters around Iceland and along the coast of northern Norway. Some Svalbard birds may also spend the winter in ice-free waters off the west coast of Svalbard.
Common eiders are maritime ducks, which occur along coastlines in the breeding season, especially on smaller islets where they are safe from mammalian predators. They breed in colonies of variable size and density. Nesting groups become established in the spring as soon as the tundra is clear of snow and the ice around the islets has melted; in most years in Svalbard this occurs towards the end of May, but may be as late as the end of June at some sites in some years.
The proportion of the population that breeds varies greatly between years depending on ice condition in spring and early summer. Common eiders leave the nesting area immediately after the eggs hatch to occupy shallows waters along the coast, where they remain until leaving Svalbard during autumn.
The common eider is subject to strong predation pressure, primarily from glaucous gulls and arctic foxes which take a large proportion of the eggs, and small chicks. The polar bear can also be a major predator of eggs at some sites.
They feed on various benthic animals; mussels are a preferred food, but they will also consume small crustaceans found in the inter-tidal zone and shallows.
Pair formation takes place during the autumn in common eiders and the same birds may stay together for several years. They are a ground-nesting species that prefers flat areas, sometimes sheltered by stones or pieces of wood, but usually the site is completely exposed. The nest is a simple, shallow scrape made by the female, but it is carefully lined with plant material and a thick layer of down. Nest-sites are re-used, and old nests are often characterized by an elevated rim.
The normal clutch size consists of four to six eggs, which are grey-green or olive-green in colour. Clutches with up to 13 eggs have been recorded in Svalbard, but large clutches are usually laid by more than one female (i.e. the result of dump nesting). The incubation period lasts 24–26 days. Before nesting the females accumulate a large reserve of fat. They do not feed during the incubation period and must therefore survive on stored reserves. They can lose up to 40 % of their body weight during incubation.
In Svalbard the common eider has a nesting strategy that differs from that of most other populations, in that the males remain with the females during the first stage of the incubation period (one to two weeks). The males then leave the nesting areas to form moulting flocks along the coast. Immediately after hatching, as soon as the young are dry, they go out to sea with their mother. The females and their young usually aggregate in groups in shallow-water areas along the coast through the summer and early fall.
The highest age recorded in Norway (including Svalbard) is 58 years.
The total common eider breeding population in Svalbard is estimated to be somewhere between 13,500 and 27,500 pairs and the late summer population is comprised of 80,000–140,000 individuals. The European breeding population is estimated to be 840,000 pairs, and is regarded as stable, although some sub-populations are declining.
Extensive collecting of down and eggs are thought to have reduced the population somewhat during the first part of the 20th century, but early numbers are rather uncertain, so this may not be the case. Although protection measures and bird sanctuaries were established in 1963 and 1973, no marked increase in the population has occurred since that time. Annual population monitoring established in the Kongsfjorden area in 1980 and surveys of moulting birds in 2002 and 2010–2011 suggest a stable population in recent decades. Human disturbance at nesting sites should be avoided in order to minimize loss of egg and chicks to predators.