Medium-sized diver between Red-throated G. stellata and Great Northern G. immer in build. Adult breeding: upperparts blackish with rectangular white spots on back, most prominent on scapulars, and white below with black and white streaks on sides of upper breast. Head and neck grey, palest on hindneck, with black and white vertical stripes on sides of neck bordering black throat patch. Adult non-breeding: Dark grey-brown on head and upperparts clearly demarcated from white throat and underparts. Crown and nape are paler than back (unlike Great Northern) and flanks often show a white patch visible on a swimming bird. Intermediate build and bill held straight are best identifying features but very difficult to separate from Pacific Diver and may not be possible with non-breeding birds.
Breeds beside medium-large freshwater lakes, often with islands. On passage and in winter moves to coastal areas and occurs on estuaries, bays and sheltered seas.
Gavia immer has a predominantly North American breeding range, but also breeds in the European Arctic. Its European breeding population is very small (as few as 700 pairs), but was stable between 1970-1990. Trends were not available for Greenland and Iceland during 1990-2000, but there is no evidence to suggest that the species declined. Although the size of the European population could render it susceptible to the risks affecting small populations, it is marginal to a much larger non-European population.
Fish, also molluscs and crustaceans caught during 45-120 second dives
This species has a large range, with an estimated global Extent of Occurrence of 10,000,000 km˛. It has a large global population estimated to be 130,000-2,000,000 individuals (Wetlands International 2002). Global population trends have not been quantified, but the species is not believed to approach the thresholds for the population decline criterion of the IUCN Red List (i.e. declining more than 30% in ten years or three generations). For these reasons, the species is evaluated as Least Concern. [conservation status from birdlife.org]
Breeds early-May to September, begins mid-June in north of range. Nests beside water often on an island or spit, a shallow scrape or more rarely a substantial mound of vegetation built in shallow water. Eggs: 2 (1 in replacement clutch), olive-brown, sometimes more greenish with sparse black blotches (84 x 53mm). Incubated by both sexes for 28-29 days. Young tended by both sexes, feed themselves at 35 days and fly at about 60 days
Migratory and dispersive. Some Scottish breeding lochs deserted August, but movement to salt water chiefly September-October; return about April. In Lapland and Russia, timing of movements to and from breeding lakes associated with thawing and freezing of fresh waters; spring return faster. Some reach Baltic by September and most at sea by mid-October. Main departures from Baltic and Black Sea winter quarters from mid-April to mid-May