Black-throated loon: Difference between revisions

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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 Template:Convert in length and can weigh anywhere from Template:Convert. In breeding plumage, the adult of the nominate subspecies has mostly black upperparts, with the exception of some of the Template:Birdgloss and Template:Birdgloss, 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 Template:Birdgloss. 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 Template:Convert 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 27Template:Nbspto 29Template:Nbspdays, 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 Template:Convert. 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.<ref>Template:Cite book</ref> Linnaeus specified the type locality as Europe and America but in 1761 he restricted it to Sweden.<ref>Template:Cite book</ref><ref>Template:Cite book</ref> 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.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> The genus name Template:Lang comes from the Latin for Template:Gloss, as used by the ancient Roman naturalist Pliny the Elder.<ref name=Johnsgard/> The specific Template:Lang is Latin for Template:Gloss or Template:Gloss.<ref name= job90>Template:Cite book</ref> The name of the subspecies viridigularis stems from the Latin Template:Lang, meaning Template:Gloss, and the Latin Template:Lang, meaning Template:Gloss, in reference to the green throat of this subspecies.<ref name="hbwname">Template:Cite web</ref> 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.<ref name="hbw"/>

There are two subspecies:<ref name=ioc>Template:Cite web</ref>

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.<ref name="hbw"/>
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.<ref name="hbw"/>

G. a. viridigularis was considered to be a separate species when described by Jonathan Dwight in 1918,<ref name="Dwight1918">Template:Cite journal</ref> but in 1919 Arthur Cleveland Bent suggested that it be moved to its current placement as a subspecies.<ref name="Bent1919">Template:Cite journal</ref> The black-throated loon was previously considered conspecific with the North American Pacific loon, which was its subspecies,<ref name="AOU"/> but they have now been split into two species;<ref name="hbw"/> there was no evidence of the two interbreeding in areas where they occurred together.<ref name="Sprengelmeyer2014"/> Furthermore, the architecture of the air sacs in the lungs of the two species are significantly different.<ref>Template:Cite journal</ref> This split was done by the AOU in 1985.<ref name="AOU">Template:Cite journal</ref> 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.<ref name="Sprengelmeyer2014">Template:Cite thesis</ref>

Description

The adult black-throated loon is Template:Convert in length with a Template:Convert wingspan and a weight of Template:Convert. The nominate subspecies in its Template:Birdgloss 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 Template:Birdgloss, 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 Template:Birdgloss and Template:Birdgloss. There are small white spots on both the lesser and median coverts. The rest of the Template:Birdgloss is a blackish colour. The Template:Birdgloss 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.<ref name="hbw">Template:Cite journal</ref> The subspecies viridigularis retains a purplish gloss, although less than the nominate.<ref name="Bent1919"/>

File:GaviaArctica.svg

Breeding, top, and non-breeding, bottom, black-throated loons

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.<ref name="hbw"/>

File:Gavia arcticaLake Zeller 02.JPG

Non-breeding adult

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.<ref name="hbw"/>

The black-throated loon can be distinguished from the Pacific loon by the white on the flanks of the former.<ref name="Floyd2008">Template:Cite book</ref>

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".<ref name="hbw"/>

Distribution and habitat

The black-throated loon has a large range, breeding taking place across northern Europe, Asia,<ref name="iucn status 13 November 2021" /> and the Seward Peninsula in Alaska.<ref name="Floyd2008"/> When breeding, it is found in the area around isolated, deep freshwater lakes<ref name="Hauber2014"/> larger than Template:Convert,<ref name="Hakeetal.2005">Template:Cite journal</ref> especially those with inlets,<ref name="hbw"/> as it prefers to face only small stretches of open water.<ref name="MudgeTalbot1992"/> When it is not breeding, this loon moves in a general southward direction and towards ice-free sea,<ref name="hbw"/> usually wintering in coasts on north-east Atlantic Ocean and those on the eastern and western Pacific Ocean, such as the coasts of Japan.<ref name="iucn status 13 November 2021" /> 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.<ref name="hbw"/>

Behaviour

File:Gavia arctica taking off.jpg

A black-throated loon taking off

Like other loons, this bird does not take flight well; it takes off by pattering on a "runway" of water.<ref name="Pielou2012">Template:Cite book</ref> While flying, it makes a barking "kwow" flight call.<ref name="PetersonMountfort2001">Template:Cite book</ref>

Breeding

File:Black throated loon family feeding on Lake Sträken, Sweden.jpg

Black-throated loon family feeding on Lake Stråken, Sweden

This species usually nests on the ground<ref name="Petersen1979">Template:Cite journal</ref> within about Template:Convert of the lake it breeds at.<ref name="Hakeetal.2005"/> This loon also sometimes nests on vegetation, like Arctophila fulva, that has emerged from lakes. The nest site is often reused the next year.<ref name="BergmanDerksen1977"/> The nest itself is oval-shaped<ref name="Petersen1979"/> and built mostly by the female<ref name="Sjolander1978"/> out of heaped plant material like leaves and sticks.<ref name="Hauber2014">Template:Cite book</ref> The nest is about Template:Convert 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 Template:Convert.<ref name="BergmanDerksen1977"/>

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,<ref name="hbw"/> when there is adequate water for it to take off.<ref name="Petersen1979"/> It usually arrives before the lake thaws, in the latter case.<ref name="Sjolander1978">Template:Cite journal</ref>

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 Template:Birdgloss itself. The female stays ashore for a maximum of about 23 minutes and usually starts to build the nest.<ref name="Sjolander1978"/>

File:Gavia arctica MWNH 1918.JPG

Eggs, Collection Museum Wiesbaden, Germany

The black-throated loon lays a clutch of two, very rarely one or three,<ref name="hbw"/> Template:Convert eggs that are brown-green with darker speckles. These eggs are incubated by both parents for a period of 27Template:Nbspto 29Template:Nbspdays,<ref name="Hauber2014"/> with the female spending the most time 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.<ref name="Sjolander1978"/> After they hatch, the mobile young are fed by both parents for a period of weeks.<ref name="Hauber2014"/> The chicks fledge about 60Template:Nbspto 65Template:Nbspdays after hatching, and achieve sexual maturity after two to three years.<ref name="hbw"/>

Nesting success, whether or not at least one chick will hatch from any given nest, is variable year to year, ranging from under 30% to over 90%.<ref name="BergmanDerksen1977"/> For clutches of two eggs, the average nesting success is about 50%, whereas in clutches with only one egg, this rate is about 60%.<ref name="MudgeTalbot1992"/> The nesting success is influenced most by predation,<ref name="Petersen1979"/> and flooding.<ref name="MudgeTalbot1992">Template:Cite journal</ref> 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.<ref name="BergmanDerksen1977">Template:Cite journal</ref> In Scotland, a study concluded that a single pair usually fledges a chick, on average, 25% of the time per year.<ref name="hbw"/> This can be increased by artificial means, such as constructing rafts for loons to nest on.<ref>Template:Cite journal</ref> 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.<ref name="Eriksson1986">Template:Cite journal</ref>

Feeding

A top predator in the pelagic zone of some subarctic lakes,<ref name="AmundsenLafferty2009">Template:Cite journal</ref> this bird feeds on fish and sometimes insects, molluscs, crustaceans, and plant matter.<ref name="hbw" /> The black-throated loon usually forages by itself or in pairs, rarely feeding in groups with multiple species.<ref name="BaltzMorejohn1977">Template:Cite journal</ref> It dives from the water,<ref name="De GraafTilghman1985">Template:Cite journal</ref> at depths of no more than Template:Convert.<ref name="Bundy2009">Template:Cite journal</ref> 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.<ref name="BNA">Template:Cite journal</ref> 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.<ref name="Bundy2009"/>

File:Parelduiker in haven en op zee-4961855.webm

A video of a black-throated loon foraging

When it is breeding, the adult usually feeds away from the nest, foraging either at the opposite end of the breeding or at lakes near the breeding lake. When foraging for newly hatched chicks, the adult forages in the lake where the nest is or in nearby lakes, returning after a prey item has been caught. When the chicks are older, they usually accompany both of the parents, swimming a few metres behind them. The strategy that predominates immediately after hatching is generally still employed when the chicks are older, but at a reduced rate.<ref name="Jackson2002">Template:Cite journal</ref> 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.<ref name="Sjolander1978" />

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 Template:Convert in length. Although in these chicks trout makes up the majority of their diet, they are still fed invertebrates in large numbers. In all lakes, salmonids make up an important part of the chicks' diet after eight days. Salmonids, especially those between Template:Convert, are important in the diets of older chicks. Eels are also an important food for older chicks.<ref name="Jackson2002" />

Predators and parasites

The black-throated loon is sometimes parasitized by Eustrongylides tubifex, a species of nematode that can cause Eustrongylidosis.<ref name="FastzkieCrites1977">Template:Cite journal</ref> Mammalian predators, such as red foxes and pine martens,<ref name="Hakeetal.2005"/> are likely the cause of about 40% of clutch losses. Avian predators, such as hooded crows, also take the eggs of this loon.<ref name="MudgeTalbot1992" />

Status

Conservation

Despite 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.<ref name="iucn status 13 November 2021" /> In the United States, it is protected under the Migratory Bird Treaty Act of 1918,<ref>Template:Cite web</ref> while in Europe and Africa, it is protected under the Agreement on the Conservation of African-Eurasian Migratory Waterbirds.<ref>Template:Cite web</ref>

Threats

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%.<ref name="hbw"/>