Behaviour Northern populations of this species are highly migratory^{3, 7}, with those breeding in the milder parts of western or southern Europe⁷ being sedentary^{1, 3, 7} or only making short-distance dispersal movements^{1, 3, 7} governed by harsh weather conditions³. The spring migration may occur as early as February in mild winters, with the main migration occurring from March to early-April³. The breeding grounds are reoccupied from the early-March (in the south) to early May (in Siberia)³ with breeding starting from April-May¹. The species mainly moults on the breeding grounds before the autumn migration, becoming flightless for a period of 3-4 weeks³. The autumn migration to wintering grounds peaks between late-September and November³, females migrating slightly later than the males (which results in a segregation of the sexes in the wintering range: males further north and females further south)⁷. The species breeds in single pairs or loose groups¹ and travels in small parties on passage⁷, sometimes gathering in flocks of many thousands during the post-breeding moult period⁷. During the winter it becomes highly gregarious, gathering in flocks of many thousands of individuals^{3, 4, 7}. The species can be crepuscular in the winter, and often feeds by night² by bottom-feeding and diving⁷ (most foraging being done at depths of 1-3 m)⁵. Habitat The species requires extensive areas of nutrient-rich open water less than 6 m deep³ that is uncluttered with floating vegetation⁷ but has abundant submerged macrophytes^{2, 4}, surrounding emergent⁴ vegetation and/or animal food (e.g. Chironomid larvae)². Breeding In its breeding range the species inhabits base-rich (e.g. saline, brackish or soda) lakes², eutrophic freshwater lakes, well-vegetated freshwater or brackish⁵ marshes with areas of open water, swamps^{1, 3} and slow-flowing rivers^{1, 3, 4}. Although it shows a strong preference for inland wetlands⁷, the species will shift to coastal habitats such as sheltered coastal bays² when driven by frost or other compelling factors⁷. Non-breeding During the winter the species frequents similar habitats to those it breeds in, including large lakes^{1, 3, 6}, slow-flowing rivers^{1, 3, 4}, reservoirs^{1, 3, 4, 6}, brackish waters, marshes, weirs (Africa)⁶ and flooded gravel pits⁹. As in the breeding season, the species will shift to coastal habitats such as brackish lagoons^{1, 3}, tidal estuaries^{1, 3, 4} and inshore waters^{3, 4} (where it associates with sewage outfalls)² when driven by frost or other compelling factors⁷. Diet The species is omnivorous, its diet consisting of seeds^{1, 2}, roots¹, rhizomes² and the vegetative parts of grasses, sedges and aquatic plants^{1, 2, 5}, as well as aquatic insects and larvae¹ (e.g. midge and caddis fly larvae during the summer)⁵, molluscs, crustaceans, worms¹ (oligochaetes)¹¹, amphibians¹ (e.g. frogs and tadpoles)⁶ and small fish¹. Breeding site The nest is a depression¹ or shallow cup² in a thick heap of vegetation¹ positioned on the ground^{1, 4} (usually within 10 m of water)⁷, in shallow water^{1, 4} (c.30 cm deep)⁵, concealed in thick waterside vegetation^{1, 4} (e.g. reedbeds)⁵, or on floating mats of reeds of other vegetation⁵. In years of high water levels when there are few emergent reedbeds or floating mats the species may nest in sedge tussocks, in flooded fields, or under bushes on hummocks⁵. Management information In the Trebon Basin Biosphere Reserve, Czech Republic, it was found that artificial islands and wide strips of littoral vegetation are the most secure breeding habitats that can be created for the species (nest survival in littoral habitats was improved by reduced nest visibility, increased water depth, and increased distance from the nest to the habitat edge, and nest survival on islands was improved with increased distance to open water)¹⁰. In the UK (Salford docks, Manchester) the species prefers to feed in waters heavily polluted with sewage that are devoid of aquatic vegetation but hold high densities of oligocheates and other pollution-tolerant organisms. The species may therefore suffer from plans to improve water quality in the docks (e.g. modernising sewage treatment systems and oxygenating water)¹¹. The cyclical removal of adult fish from an artificial waterbody (gravel pit) in the UK attracted nesting pairs to the area by causing an increase in invertebrate food availability and an increase in the growth of submerged aquatic macrophytes²⁰. The removed fish (dead or alive) were sold to generate funds²⁰.

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The species is threatened by disturbance from hunting^{1, 2, 12}, water-based recreation^{2, 9} and from machinery noise from urban development⁸. It is also threatened by habitat destruction¹ on its wintering grounds due to eutrophication (partially as a result of nutrient run-off from agricultural land)². The species suffers from nest predation by American mink Mustela vison in Poland¹³, and adults are poisoned by ingesting lead shot (Spain)¹⁴ and drowned in freshwater fishing nets with mesh sizes greater than 5 cm (China)¹⁵. The species is also susceptible to avian influenza, so may be threatened by future outbreaks of the disease¹⁷. Utilisation This species is hunted in Northern Island¹², Spain¹⁴ and Italy¹⁸, and the eggs of this species used to be (and possibly still are) harvested in Iceland¹⁶. The species is also hunted for commercial and recreational purposes in Gilan Province, northern Iran¹⁹.