Arctic Loon

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. It winters along sheltered, ice-free coasts of the north-east Atlantic Ocean and the eastern and western Pacific Ocean. This loon was first described by Carl Linnaeus in 1758 and has two subspecies. It was previously considered to be the same species as the Pacific loon, of which it is traditionally considered to be a sister species, although this is debated. In a study that used mitochondrial and nuclear intron DNA, the black-throated loon was found to be sister to a clade consisting of the Pacific loon and two sister species, the common loon and the yellow-billed loon.

The black-throated loon measures about in length and can weigh anywhere from . In breeding plumage, the adult of the nominate subspecies has mostly black upperparts, with the exception of some of the and , which have white squares. The head and hindneck are grey, and the sides white and striped black. Most of the throat is also black, giving this bird the name "black-throated loon". The colour of the throat patch can be used to distinguish the two subspecies; the throat patch of the other subspecies, ''G. a. viridigularis'', is green. The underparts are mostly white, including the bottom of the throat. The flanks are also white, a feature which can be used to separate this bird from the Pacific loon. 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 . The juvenile is similar to the non-breeding adult, except more brown overall.

The timing of the breeding season is variable; in the southern part of its range, this loon starts breeding in April, whereas in the northern portion, it waits until after the spring thaw. It builds an oval nest about across, either near the breeding lake or on vegetation emerging from it. 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 27to 29days, the chick hatches, and is fed a diet of small fish and invertebrates. The adult mostly eats fish. To catch this food, it forages by itself or in pairs, very rarely foraging in groups. It dives from the water, going no deeper than . Most dives are successful. Whether or not at least one chick will hatch from a nest is variable, ranging from 30% to 90%. Most failures come from predators and flooding. Overall, the population of this loon is declining, although the International Union for Conservation of Nature (IUCN) still rates it as least concern, because the population decline is not rapid enough. The black-throated loon is protected under both the Migratory Bird Treaty Act of 1918 and the Agreement on the Conservation of African-Eurasian Migratory Waterbirds.

Taxonomy and etymology

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''. Linnaeus specified the type locality as Europe and America but in 1761 he restricted it to Sweden. 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. This proposal was adopted by the American Ornithologists' Union (AOU) in 1899. The genus name comes from the Latin for , as used by the ancient Roman naturalist Pliny the Elder. The specific is Latin for or . The name of the subspecies ''viridigularis'' stems from the Latin , meaning , and the Latin , meaning , in reference to the green throat of this subspecies. The common name, black-throated loon, stems from its black throat patch. This loon is also called the Arctic loon and the black-throated diver.

There are two subspecies:
*''Gavia arctica arctica'' (Linnaeus, 1758) – This subspecies is found in northern Europe, east to the center of northern Asia, and from that to the Lena River and Transbaikal. It migrates to the coasts of northwestern Europe and the coasts of the Mediterranean, Black, and Caspian Seas.
*''G. a. viridigularis'' Dwight, 1918 – This subspecies is found in eastern Russia from the Lena River and Transbaikal east to the peninsulas of Chukotka and Kamchatka and the northern portion of Sakhalin. It migrates to the northwestern Pacific coasts.

''G. a. viridigularis'' was considered to be a separate species when described by Jonathan Dwight in 1918, but in 1919 Arthur Cleveland Bent suggested that it be moved to its current placement as a subspecies. The black-throated loon was previously considered conspecific with the North American Pacific loon, which was its subspecies, but they have now been split into two species; there was no evidence of the two interbreeding in areas where they occurred together. Furthermore, the architecture of the air sacs in the lungs of the two species are significantly different. This split was done by the AOU in 1985. The phylogeny of this species is debated, the black-throated loon and the Pacific loon traditionally being considered sister species, whereas a study using mitochondrial and nuclear intron DNA supported placing the black-throated loon sister to a clade consisting of the Pacific loon and the two sister species that are the common loon and the yellow-billed loon. This latter study is criticised on the basis that it may form a phylogeny on incomplete lineage sorting. In the former phylogeny, the split between the Pacific loon and the black-throated loon is proposed to have happened about 6.5 million years ago.

Description

The adult black-throated loon is in length with a wingspan and a weight of . The nominate subspecies in its has a grey head and hindneck, with a black throat and a large black patch on the foreneck, both of which have a soft purple gloss. The lower throat has a necklace-shaped patch of short parallel white lines. The sides of the throat have about five long parallel white lines that start at the side of the patch on the lower throat and run down to the chest, which also has a pattern of parallel white and black lines. The rest of the , including the centre of the chest, are pure white. 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 and . There are small white spots on both the lesser and median coverts. The rest of the is a blackish colour. The is paler than the upperwing, and the underwing coverts are white. The tail is blackish. The bill and legs are black, with a pale grey colour on the inner half of the legs. The toes and the webs are grey, the latter also being flesh coloured. The irides are a deep brown-red. 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 subspecies ''viridigularis'' retains a purplish gloss, although less than the nominate.

The non-breeding adult differs from the breeding adult in that the cap and the back of the neck are more brownish. The non-breeding adult also lacks the patterned upperparts of the breeding adult, although some of the upperwing coverts do not lose their white spots. This results in the upperparts being an almost unpatterned black from above. The sides of the throat are usually darker at the white border separating the sides of the throat and the front of the throat; most of the time a thin dark necklace between these two areas can be seen. There is white on the sides of the head that are below the eye. The bill is a steel-grey with, similar to the breeding adult, a blackish tip.

The juvenile is similar to the non-breeding adult, but has a browner appearance. It has a buffy scaling on the upperparts that is especially pronounced on the scapulars. The lower face and front of the neck has a diffused brownish tinge. 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. The abdomen is pale.

The black-throated loon can be distinguished from the Pacific loon by the white on the flanks of the former.

Vocalizations

The male, when breeding, vocalizes a loud and rhythmic "oooéé-cu-cloooéé-cu-cloooéé-cu-cluuéé" whistling song. A "áááh-oo" wail can also be heard, and a growling or croaking "knarr-knor", a sound given especially at night. The alarm call at the nest is a rising "uweek".

Distribution and habitat

The black-throated loon has a large range, breeding taking place across northern Europe, Asia, and the Seward Peninsula in Alaska. When breeding, it is found in the area around isolated, deep freshwater lakes larger than , especially those with inlets, as it prefers to face only small stretches of open water. When it is not breeding, this loon moves in a general southward direction and towards ice-free sea, usually wintering in coasts on north-east Atlantic Ocean and those on the eastern and western Pacific Ocean, such as the coasts of Japan. 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.

Behaviour

Like other loons, this bird does not take flight well; it takes off by pattering on a "runway" of water. While flying, it makes a barking "kwow" flight call.

Breeding

This species usually nests on the ground within about of the lake it breeds at. This loon also sometimes nests on vegetation, like ''Arctophila fulva

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