Known as the Long-tailed Duck in Europe, and until recently as the Oldsquaw in North America, this midsize sea duck is a true arctic species, breeding in tundra and taiga regions around the globe, as far as 80°N. After breeding, it migrates to cold and temperate-water coasts of North America, western Greenland, eastern Asia, and the Great Lakes. Individuals dive for food, to impressive depths not reached by other sea ducks (more than 60 meters), and have a broad diet of animal prey, focusing on food items that are locally abundant.

Unlike other waterfowl, Long-tailed Ducks have 3 plumages, instead of the normal 2 ( Salomonsen, F. (1949a). Some notes on the moult of the Long-tailed Duck. Avic. Mag. 55:59-62. Close Salomonsen 1949a). Additionally, the male Alternate plumage is worn only in winter, while the Basic plumage is acquired in spring and worn for the breeding season. The most distinctive feature, one that gives this species its name, is the 2 slim and elongated central tail feathers that stream behind the male. In summer, white feathers on the head of the male are replaced with black, and the male in breeding condition shows a bright-pink band around his bill. A distinctive feature of this bird is its call: nasal-sounding and audible from quite a distance along the coasts of its wintering grounds and on its tundra breeding grounds.

Despite its wide distribution, relatively little research has been devoted to this species. For behavior and reproductive ecology, studies by Alison ( Alison, R. M. (1973). Delayed nesting in Oldsquaws. Bird-Banding 44:61-62. Close Alison 1973, Alison, R. M. (1975a). Breeding biology and behavior of the Oldsquaw (Clangula hyemalis L.). Ornithological Monographs 18:1-52. Close Alison 1975a, Alison, R. M. (1975b). Capturing and marking Oldsquaws. Bird-Banding 46:248-250. Close Alison 1975b, Alison, R. M. (1976). Oldsquaw brood behavior. Bird-Banding 47:210-213. Close Alison 1976, Alison, R. M. (1977). Homing of subadult Oldsquaws. Auk 94:383-384. Close Alison 1977), Pehrsson ( Pehrsson, O. (1986). Duckling production of the Oldsquaw in relation to spring weather and small-rodent fluctuations. Canadian Journal of Zoology 64:1835-1841. Close Pehrsson 1986), and Pehrsson and Nyström ( Pehrsson, O. and K. G. K. Nyström. (1988). Growth and movements of Oldsquaw ducklings in relation to food. Journal of Wildlife Management 52:185-191. Close Pehrsson and Nyström 1988) are comprehensive. Timing of migration and migration monitoring are described by Johnson and Richardson ( Johnson, S. R. and W. J. Richardson. (1982). Waterbird migration near the Yukon and Alaskan coast of the Beaufort Sea: II. Moult migration of seaducks in summer. Arctic 35 (2):291-301. Close Johnson and Richardson 1982), Woodby and Divoky ( Woodby, D. A. and G. J. Divoky. (1982). Spring migration of eiders and other waterbirds at Point Barrow, Alaska. Arctic 35:403-410. Close Woodby and Divoky 1982), and Alexander et al. ( Alexander, S. A., D. M. Ealey and S. J. Barry. (1988). Spring migration of eiders, Oldsquaws and Glaucous Gulls along offshore leads of the Canadian Beaufort Sea. Edmonton, AB: Can. Wildl. Serv., Western and Northern Region. Close Alexander et al. 1988, Alexander, S. A., D. L. Dickson, and S. E. Westover (1997). Spring migration of eiders and other waterbirds in offshore areas of the western Arctic. In King and Common Eiders of the Western Canadian Arctic (D. L. Dickson, Editor), Canadian Wildlife Service Occasional Paper 94. Environment Canada, Canadian Wildlife Service, Ottawa, ON, Canada. pp. 6–20. Close Alexander et al. 1997). On the wintering grounds, feeding ecology has been studied by Peterson and Ellarson ( Peterson, S. R. and R. S. Ellarson. (1977). Food habits of Oldsquaws wintering on Lake Michigan. Wilson Bulletin 89:81-91. Close Peterson and Ellarson 1977), Vermeer and Levings ( Vermeer, K. and C. D. Levings. (1977). Populations, biomass and food habits of ducks on the Fraser River Delta intertidal area, British Columbia. Wildfowl 28:49-60. Close Vermeer and Levings 1977), Johnson ( Johnson, S. R. (1984b). "Prey selection by Oldsquaws in a Beaufort Sea lagoon, Alaska." In Marine birds: their feeding ecology and commercial fisheries relationships., edited by G. A. Sanger and P. F. Springer, 12-19. Ottawa, ON: Can. Wildl. Serv. Spec. Publ. Close Johnson 1984b), Sanger and Jones ( Sanger, G. A., and R. D. Jones, Jr. (1984). Winter feeding ecology and trophic relationships of Oldsquaws and White-winged Scoters on Kachemak Bay, Alaska. In Marine Birds: Their Feeding Ecology and Commercial Fisheries Relationships (G. A. Sanger and P. F. Springer, Editors). Canadian Wildlife Service Special Publication, Ottawa, Ontario, Canada. pp. 20-28. Close Sanger and Jones 1984), Goudie and Ankney ( Goudie, R. I. and C. D. Ankney. (1986). Body size, activity budgets, and diets of sea ducks wintering in Newfoundland. Ecology 67:1475-1482. Close Goudie and Ankney 1986), and Jamieson et al. ( Jamieson, S. E., G. J. Robertson and H. G. Gilchrist. (2001). Autumn and winter diet of Long-tailed Ducks wintering in the Belcher Islands, Nunavut, Canada. Waterbirds 23:129-132. Close Jamieson et al. 2001) and nutrient reserve dynamics by Peterson and Ellarson ( Peterson, S. R. and R. S. Ellarson. (1979). Changes in Oldsquaw weight. Wilson Bulletin 91:288-300. Close Peterson and Ellarson 1979) and Leafloor et al. ( Leafloor, J. O., J. E. Thompson and C. D. Ankney. (1996). Body mass and carcass composition of fall migrant Oldsquaws. Wilson Bulletin 108:567-572. Close Leafloor et al. 1996). Contaminants have been examined by Peterson and Ellarson ( Peterson, S. R. and R. S. Ellarson. (1975). Incidence of body shot in Lake Michigan Oldsquaws. Journal of Wildlife Management 39:217-219. Close Peterson and Ellarson 1975, Peterson, S. R. and R. S. Ellarson. (1976). Total mercury residues in livers and eggs of Oldsquaws. Journal of Wildlife Management 40:704-709. Close Peterson and Ellarson 1976, Peterson, S. R. and R. S. Ellarson. (1978b). p, p'-DDE, polychlorinated biphenyls, and endrin in Oldsquaws in North America, 1969-73. Pesticides Monitoring Journal 11:170-181. Close Peterson and Ellarson 1978b).

The Long-tailed Duck is hunted throughout its range, by native hunters in the North and by recreational hunters in southern Canada and the northern United States. Obtaining population size and trend estimates for this species have proven difficult, as it usually winters in flocks of various sizes, is widely distributed, and can be far offshore. Available data for the west coast of North America suggest that this species is declining drastically. On the east coast, trends do not show evidence of declines, but estimates have been difficult to obtain.