The Long-tailed duck is a medium-sized sea duck, the only living member of its genus. Adults have white underparts, though the rest of the plumage goes through a complex molting process. The male has a long pointed tail and a dark grey bill crossed by a pink band. In winter, the male has a dark cheek patch on a mainly white head and neck, a dark breast, and mostly white body. In summer, the male is dark on the head, neck, and back with a white cheek patch. The female has a brown back and a relatively short pointed tail. In winter, the female's head and neck are white with a dark crown. In summer, the head is dark. Juveniles resemble adult females in autumn plumage, though with a lighter, less distinct cheek patch.
Long-tailed ducks are found in the North Atlantic region, Alaska, northern Canada, northern Europe, and Russia. Their breeding habitats include tundra pools and marshes, sea coasts, large mountain lakes, bogs, and pools of standing water. They are migratory and winter along the eastern and western coasts of North America, on the Great Lakes, coastal northern Europe and Asia, with stragglers to the Black Sea. Their most important wintering area is the Baltic Sea. Long-tailed ducks winter at sea, but also in saline, brackish or fresh estuarine waters, brackish lagoons or on large freshwater lakes.
Long-tailed ducks are gregarious birds that form large flocks in winter and during migration. They are active during the day and spend most of the time foraging by diving and swimming underwater. These birds usually feed close to the surface but may also dive to depths of 60 m (200 ft). They are the only ducks that use their wings to dive, which gives them the ability to dive much deeper than other ducks. Long-tailed ducks communicate with the help of various calls including growling, clucking, squawking, and yodeling sounds. The males are loud and vocal and have a musical yodeling call "ow, ow, owal-ow".
Long-tailed ducks are carnivores and feed mainly on mollusks, crustaceans and some small fish. On breeding grounds, ducks consume aquatic insects, also crustaceans and mollusks, fish eggs, and some plant material.
Long-tailed ducks are serially monogamous; pair bonds may last for a single breeding season or may re-form for several following years. The birds may breed in single pairs or in loose groups and breeding usually takes place in spring. During this time males perform courtship displays, raising tails, bowing and pulling their heads back and produce deep calls. Long-tailed ducks nest on the ground usually close to the water. Females lay 5-9 eggs which they incubate alone around 24-29 days while males leave for molting. Shortly after hatching, ducklings leave the nest and follow their mother to the nearest freshwater pond where they learn how to get food. They fledge 35-40 days after hatching and become reproductively mature at 2 years of age.
The main threats to Long-tailed ducks include heavy bycatch mortality in gillnet fisheries and oil pollution. These birds also suffer hunting pressure over some Arctic regions during their migration. Degradation and drainage of wetlands are important threats too.
According to the IUCN Red List, the total Long-tailed duck population size is around 3,200,000-3,750,000 individuals. Currently, this species is classified as Vulnerable (VU) on the IUCN Red List and its numbers today are decreasing.