AppearanceThe adult black-throated loon is 58 to 73 cm in length with a 100 to 130 cm wingspan and a weight of 1.3 to 3.4 kilograms. The nominate subspecies in its breeding plumage 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 underparts, 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 mantle and scapulars. There are small white spots on both the lesser and median coverts. The rest of the upperwing is a blackish colour. The underwing 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.
NamingThere are two subspecies:
⤷ ''Gavia arctica arctica'' – 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.
DistributionThe black-throated loon has a large range, breeding taking place across northern Europe, Asia, and the Seward Peninsula in Alaska.
StatusDespite the fact that its population is declining, the black-throated loon is listed as a species of least concern by the IUCN. This is because the species has a large population and an extremely large range, and its decline does not appear to be rapid. In North America, it is protected under the Migratory Bird Treaty Act of 1918, while in Europe and Africa, it is protected under the Agreement on the Conservation of African-Eurasian Migratory Waterbirds.
BehaviorLike other loons, this bird takes off by pattering on a "runway" of water. While flying, it makes a barking "kwow" flight call.
ReproductionThis species usually nests on the ground within about 1 metre of the lake it breeds at. This loon also sometimes nests on vegetation, like ''Arctophila fulva'', that have emerged from lakes. The nest site is often reused the next year. The nest itself is oval-shaped and built mostly by the female out of heaped plant material like leaves and sticks.
The nest measures about 23 centimetres 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 150 metres.
In the southern portion of its range, this loon starts to breed in April, whereas in the northern parts of its range, it waits until the spring thaw, when there is adequate water for it to take off in. It usually arrives before the lake thaws, in the latter case.
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 preens itself. The female stays ashore for a maximum of about 23 minutes and usually starts to build the nest.
The black-throated loon lays a clutch of two, very rarely one or three, 76 by 47 millimetres eggs that are brown-green with darker speckles. These eggs are incubated by both parents for a period of 27 to 29 days, with the female spending the most time out of the sexes incubating.
During incubation, this bird turns its eggs. The interval between when they are turned is very irregular, ranging from one minute to about six hours. After they hatch, the mobile young are fed by both parents for a period of weeks. The chicks fledge about 60 to 65 days after hatching, and achieve sexual maturity after two to three years.
FoodA top predator in the pelagic zone of some subarctic lakes, this bird feeds on fish and sometimes insects, molluscs, crustaceans, and plant matter. The black-throated loon usually forages by itself or in pairs, rarely feeding in groups with multiple species. It dives from the water, at depths of no more than 5 metres. Just before diving, this loon stretches and holds up its neck until it is erect and at full length.
PredatorsThe black-throated loon is sometimes parasitized by ''Eustrongylides tubifex'', a species of nematode that can cause Eustrongylidosis. Mammalian predators, such as red foxes and pine martens, are likely the cause of about 40% of clutch losses. Avian predators, such as hooded crows, also take the eggs of this loon.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%.
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