Blackthroated Loon

The black-throated loon, also known as the Arctic loon or black-throated diver, is an aquatic migratory bird found across the Northern Hemisphere. It breeds predominantly in freshwater lakes of northern Europe and Asia and winters along sheltered, ice-free coastal regions of the northeast Atlantic and both sides of the Pacific Ocean. First described by Carl Linnaeus in 1758, the species comprises two recognised subspecies and occupies an important place within the genus Gavia, which contains all loons and divers.

Taxonomy and Etymology

Carl Linnaeus originally named the species Colymbus arcticus in the 10th edition of Systema Naturae (1758), restricting its type locality to Sweden in 1761. Subsequent taxonomic revision, particularly by Joel Asaph Allen in 1897, led to its placement within the genus Gavia, a change adopted by the American Ornithologists’ Union in 1899. The name Gavia originates from Latin descriptions of seabirds recorded by Pliny the Elder, while the specific epithet arctica means “of the north”.
Two subspecies of the black-throated loon are widely recognised:

• Gavia arctica arctica, found from northern Europe eastwards across the Palearctic to the Lena River and Transbaikal, wintering around the coasts of northwestern Europe, the Mediterranean, the Black Sea, and the Caspian Sea.
• Gavia arctica viridigularis, occurring from the Lena River eastwards to Chukotka, Kamchatka, and northern Sakhalin, wintering in the northwestern Pacific. This subspecies, described in 1918, was initially considered a distinct species due to its green throat patch, though later research placed it correctly as a subspecies.

Historically, the black-throated loon was considered conspecific with the Pacific loon, but in 1985 the AOU split the two based on evidence including genetic divergence, anatomical differences in lung air-sac structures, and lack of interbreeding in overlap zones. Mitochondrial DNA and intron studies suggest a debated phylogeny, with some analyses placing G. arctica as sister to a clade containing the Pacific, common, and yellow-billed loons.

Physical Description

The adult black-throated loon is approximately 58–77 cm long, with a wingspan often ranging from 100–130 cm, and weight varying between about 1.5 and 3.4 kg depending on season and region. In breeding plumage, adults of the nominate subspecies have grey heads and hindnecks, a black throat with a purple sheen, and a large black foreneck patch bordered with short, necklace-like white stripes. Additional long white stripes run parallel down the sides of the neck toward the chest. The underparts, including the lower throat and belly, are bright white.
The upperparts display a blackish tone with characteristic contrasting white squares on parts of the scapulars and wing coverts. Smaller white spots may occur on the lesser and median coverts. The underwing coverts are white, and the tail is blackish. The bill is slender, black, and pointed; the legs are blackish with pale grey on their inner surfaces.
The subspecies viridigularis closely resembles the nominate form but replaces the black throat patch with a green one and may show less purple gloss. Nonbreeding adults lose the distinctive neck and upperpart patterning, presenting a more uniformly brownish-black appearance. Some small white spots on the wing coverts may remain. Juveniles are browner still, with buff scaling on the upperparts and an overall duller tone.
Distinctive field marks include the white flanks—absent in the Pacific loon—which provide a key identification feature.

Vocalisations

This loon produces a variety of calls, many of which are associated with breeding. Males emit a rhythmic, whistling song transcribed as oo-oo-cuc-loo, often repeated in territorial or courtship contexts. Other sounds include a drawn-out hoo wail and a grumbling knarr or knor, frequently heard at night. At the nest, adults give a rising uweek alarm call.

Distribution and Habitat

The species’ breeding range spans northern Europe and Asia, including regions of Scandinavia, Russia, and the Seward Peninsula of Alaska. Breeding typically occurs around isolated deep freshwater lakes, particularly those with inlets and sheltered environments that reduce large open-water exposure. Waterbodies used for nesting tend to be larger than those preferred by some related loon species.
During winter, the black-throated loon migrates to ice-free coastal waters of the northeast Atlantic and the Pacific. These include sheltered bays, fjords, and coastal inlets where fish availability remains high.

Breeding Ecology

Breeding timing varies by latitude: in southern areas, nesting can begin in April, whereas in the far north it commences after ice breakup. Nests are oval structures usually placed near the water’s edge or on emergent vegetation. They measure roughly 30–40 cm across and are constructed from local plant materials.
Typical clutches contain two brownish-green eggs marked with dark blotches, though one or three eggs may occur. The incubation period lasts 27–29 days and is carried out by both parents. Chicks hatch covered in brownish down and are fed initially with small fish and freshwater invertebrates. The adults themselves feed mainly on fish, obtained through underwater pursuit. Most foraging dives remain shallower than 5 m, and a high proportion of dives result in prey capture.
Reproductive success varies widely across the range, with reported nesting success between 30% and 90%. Nest losses stem primarily from predation and flooding events.

Behaviour and Feeding

Black-throated loons forage alone or in pairs, seldom forming larger feeding groups. Their diet consists almost exclusively of fish, with invertebrates more commonly fed to chicks. They are adept divers, propelling themselves underwater using powerful feet set far back on the body. While highly aquatic, they must run along the water surface to gain sufficient momentum for takeoff.
Migration occurs seasonally, often in small groups. On the wintering grounds they frequent coastal waters but may occasionally enter estuaries and brackish zones.