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A migratory aquatic bird found in the northern hemisphere

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 it 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 shaped nest that measures 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. This contrasts with the mostly fish diet of the adult. 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.

The black throated loon, Gavia arctica, was originally described by Carl Linnaeus as Colymbus arcticus in his 18th century work, Systema Naturae. The genus name Gavia comes from the Latin for "sea mew", as used by the ancient Roman naturalist Pliny the Elder. The name of the subspecies viridigularis stems from the Latin "viridis", meaning "green", and the Latin "gularis", meaning "throated", 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. but a year after, in 1919, Arthur Cleveland Bent suggested that the former species 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, 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 criticized 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.

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 does still retain a purplish gloss, although it is less than the nominate.

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". especially those with inlets, While flying, it makes a barking "kwow" flight call.

This species usually nests on the ground within about of the lake it breeds at. The nest measures about across. Families of black throated loons often move their nest site from the original nest ponds they inhabited to wetlands nearby after the chicks reach two weeks of age. The journey is generally less than
Before copulation, the female hunches its neck and swims close to the shore until it finds a suitable place and then lies down on the shore. The male sometimes adopts the same posture as the female. During this time, the only vocalization made is a one note "hum". During copulation, the male, coming ashore, mounts the female and occasionally flaps its wings loudly. After this, the male returns to the water and itself. The female stays ashore for a maximum of about 23 minutes and usually starts to build the nest. Some of the adults that lose their clutch early in the incubation period renest. Most of the time, only one chick survives to fledge, the other dying within seven days of hatching. In Scotland, a study concluded that a single pair usually fledges a chick, on average, 25% of the time per year. Whether or not there is at least one chick fledged is influenced by the density of fish in the breeding lake; a lake with a higher density of fish usually reduces the chance that a pair will fledge a chick, even though this loon feeds mainly on fish. There are two factors that might contribute to this; the first being that aquatic insects, an alternative food source for chicks, are more dense when there are less fish, and the second being that a higher density of fish means more northern pike, a predator of small chicks.

A top predator in the pelagic zone of some subarctic lakes, this bird feeds on fish and sometimes insects, molluscs, crustaceans, and plant matter. It dives from the water, at depths of no more than Just before diving, this loon stretches and holds up its neck until it is erect and at full length. It usually jumps slightly upwards before diving. These dives are frequent, with an average of about 1.6 dives per minute. Most dives, about 80% of them, are successful, and those that are successful are usually shorter than those that are unsuccessful, with an average of 17 seconds for each successful dive, and 27 seconds for each unsuccessful dive. 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 chicks are fed only one item of prey at a time. The young are also able to capture food themselves at least 36 days after hatching, although they are still fed daily up until about 70 days of age. Mammalian predators, such as red foxes and pine martens, while in Europe and Africa, it is protected under the Agreement on the Conservation of African Eurasian Migratory Waterbirds.

Acidification and heavy metal pollution of the breeding lake possibly threatens this bird. It is also vulnerable to oil pollution, especially when near fishing grounds. Fishing nets are also a cause of mortality. This loon is sensitive to windfarms near the coast. Overall, the annual mortality rate of the adult black throated loon is 10%. >