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The brent goose is somewhat smaller than the barnacle goose. Brent geese normally do not fly in the typical V-formation used by other geese, but in irregular masses or lines.
There are several subspecies of brent geese, each of which has somewhat different plumage. The Svalbard population belongs to the light-bellied sub-species Branta bernicla hrota.
The brent goose is somewhat smaller than the barnacle goose. The sexes are identical in appearance. Adult birds are 55–60 cm long and weigh 1.1–1.6 kg.
They have a black head, neck and breast, with white patches on the sides of the neck. The upper-parts of the body and the wings are dark grey-brown; the under-parts are light grey-brown in B. b. hrota (but dark brown in the dark-bellied subspecies B. b. bernicla). The bill and legs are black.
Juveniles resemble the adults, though they lack the white neck patches and they have more mottled plumage.
Brent geese normally do not fly in the typical V-formation used by other geese, but in irregular masses or lines.
Their call is a growling “rruk”.
Lyden er et gurglende eller knurrende«prråk».
Brent geese have a holarctic distribution. Light-bellied brent geese nest in northeast Canada, northeast Greenland, Svalbard and Franz Josef Land, while the dark-bellied sub-species nests in the tundra of Siberia east to Taymyr in Russia. East of Taymyr and in the north-western parts of arctic North America, a third sub-species is found, B. b. nigricans.
Birds from the Kile area in northeast Greenland aggregate with birds from Svalbard and Franz Josef Land to winter in Denmark and in Northumberland in England. The individuals from the rest of Greenland spend the winter in Ireland, while the dark-bellied sub-species from western Siberia winters along the North Sea coasts from Denmark to southern England and western France.
Most brent geese in Svalbard breed on Tusenøyane, south of Edgeøya. Some pairs also breed on small islands along the western and northern coast of Spitsbergen -Moffen in the north being the most important of these islands. A few pairs also breed along the coast of Spitsbergen.
Brent geese arrive at their nesting sites toward the end of May or in early June. The autumn migration occurs in the first half of September.
Relatively little is known about the biology of this species in Svalbard. They nest on flat tundra areas near ponds and lakes and on islands.
During migration and in the winter areas brent geese are more strongly associated with the coast than the other Svalbard geese and they often spend time in delta areas or in shallow-water marine areas.
In the breeding season the brent geese in Svalbard feed on various land plants such as scurvy-grass (Cochlearia) and mosses, though they will also feed on various algae in sea-water. During the winter months eelgrass and algae are their main diet. Polar bears, arctic foxes, glaucous gulls and arctic skuas all take eggs and chicks of the brent goose in Svalbard
Brent geese nest in single pairs or in loose colonies. In Svalbard there appears to be competition with the more dominant barnacle goose for nesting sites. The nest is a shallow depression, usually placed on an elevation in the terrain; the nest is lined with plant material and down.
Egg laying usually occurs in mid-June. The three to five eggs are yellow-white and are incubated by the female for 24–26 days. The male remains close to the nest at all times.
The nest is abandoned soon after the chicks hatch and the young are fledged approximately six weeks later. Right around the time the young fledge the adults moult their plumage leaving them flightless for several weeks. In this period they are very timid and highly sensitive to disturbance. Similar to the other species of geese in Svalbard the parents stay with their young through winter and until the spring migration the following year.
Sexual maturity is reached at two or three years of age.
The Svalbard population of the light-bellied brent goose is the smallest discrete migratory goose population in the world. Annual counts in their wintering areas in Denmark and England suggest a total population of about 6,800 individuals (2013). Exactly what proportion of these birds breed in Svalbard is not clear. Recent studies indicate that the Svalbard birds produce few young, and that recruitment to the population is poor. Possible competition with the barnacle goose in the breeding areas, and lack of food in the wintering areas, may hamper population growth.
Historically, the brent goose was probably the most numerous goose species in Svalbard. It was thought to number in excess of 50,000 individuals. It was widely distributed on islands all along the west coast of Spitsbergen, as well as along the coasts of the rest of the archipelago.
The population declined dramatically during the first part of the 20th century, probably due to intensive harvest of down and eggs in the breeding areas, as well as a lack of their staple food — eelgrass Zostera spp. — at their wintering grounds.