Colymbus arcticus Linnaeus, 1758 The black-throated loon (Gavia arctica), also known as the Arctic loon and the black-throated diver, is a migratory aquatic bird found in the northern hemisphere, primarily breeding in freshwater lakes in northern Europe and Asia.

In breeding plumage, the adult of the nominate subspecies has mostly black upperparts, with the exception of some of the mantle and scapulars, which have white squares.

When the bird is not breeding, the black patch on the throat is absent, replaced with white; most of the black lines on the throat are also missing, except those on the bottom sides, and the upperparts are unpatterned with the exception of a few white spots on the upperwing.

The black-throated loon usually lays a clutch of two, rarely one or three, brown-green eggs with dark splotches.

After an incubation period of 27 to 29 days, the chick hatches, and is fed a diet of small fish and invertebrates.

The black-throated loon was formally described in 1758 by the Swedish naturalist Carl Linnaeus in the tenth edition of his Systema Naturae under the binomial name Colymbus arcticus.

[3][4] In 1897 the American ornithologist Joel Asaph Allen suggested that the black-throated loon should be moved to the genus Gavia that had been introduced by Johann Forster in 1788.

[5][6] The genus name Gavia comes from the Latin for 'sea mew', as used by the ancient Roman naturalist Pliny the Elder.

The upperparts are blackish down to the base of the wing, where there are a few rows of high contrast white squares that cover the mantle and scapulars.

The sexes are alike, and the subspecies viridigularis is very similar to the nominate except that the former has a green throat patch, instead of black.

The juvenile does not have the white spots on the wing coverts, and its irides are darker and more dull in colour.

The chick hatches with down feathers that range from sooty-brown to brownish-grey, usually with a slightly paler head.

[10] The black-throated loon has a large range, breeding taking place across northern Europe, Asia,[1] and the Seward Peninsula in Alaska.

[17] When breeding, it is found in the area around isolated, deep freshwater lakes[18] larger than 0.1 square kilometres (0.039 sq mi),[19] especially those with inlets,[10] as it prefers to face only small stretches of open water.

[1] During this time, its habitat is usually inshore waters along sheltered coasts, although it will sometimes be found inland, in places such as the Mediterranean and Black seas.

[24] The nest itself is oval-shaped[23] and built mostly by the female[25] out of heaped plant material like leaves and sticks.

During copulation, the male, coming ashore, mounts the female and occasionally flaps its wings loudly.

[25] The black-throated loon lays a clutch of two, very rarely one or three,[10] 76 by 47 millimetres (3.0 by 1.9 in) eggs that are brown-green with darker speckles.

[24] In Scotland, a study concluded that a single pair usually fledges a chick, on average, 25% of the time per year.

[27] A top predator in the pelagic zone of some subarctic lakes,[28] this bird feeds on fish and sometimes insects, molluscs, crustaceans, and plant matter.

[10] The black-throated loon usually forages by itself or in pairs, rarely feeding in groups with multiple species.

These dives usually only result in small items, and those that yield larger pieces of food are usually more than 40 seconds, where this bird catches quick-swimming fish.

The strategy that predominates immediately after hatching is generally still employed when the chicks are older, but at a reduced rate.

[25] The diet of black-throated loon chicks varies, the prey in the breeding lake being a major factor.

For the first eight days, chicks are usually fed three-spined sticklebacks and common minnows if they are found in the breeding lake.

If they are not present, then the chicks are brought up mainly on small invertebrates until about eight days, when they are able to take trout of about 100 millimetres (3.9 in) in length.

Although in these chicks trout makes up the majority of their diet, they are still fed invertebrates in large numbers.

[20] Despite the fact that its population is declining, the black-throated loon is listed as a species of least concern by the IUCN.