| Habitat and Ecology: |
Behaviour Northern populations of this species are highly migratory (Scott and Rose 1996, Snow and Perrins 1998), with those breeding in the milder parts of western or southern Europe (Snow and Perrins 1998) being sedentary (del Hoyo et al. 1992, Scott and Rose 1996, Snow and Perrins 1998) or only making short-distance dispersal movements (del Hoyo et al. 1992, Scott and Rose 1996, Snow and Perrins 1998) governed by harsh weather conditions (Scott and Rose 1996). The spring migration may occur as early as February in mild winters, with the main migration occurring from March to early-April (Scott and Rose 1996). The breeding grounds are reoccupied from the early-March (in the south) to early May (in Siberia) (Scott and Rose 1996) with breeding starting from April-May (del Hoyo et al. 1992). The species mainly moults on the breeding grounds before the autumn migration, becoming flightless for a period of 3-4 weeks (Scott and Rose 1996). The autumn migration to wintering grounds peaks between late-September and November (Scott and Rose 1996), 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) (Snow and Perrins 1998). The species breeds in single pairs or loose groups (del Hoyo et al. 1992) and travels in small parties on passage (Snow and Perrins 1998), sometimes gathering in flocks of many thousands during the post-breeding moult period (Snow and Perrins 1998). During the winter it becomes highly gregarious, gathering in flocks of many thousands of individuals (Madge and Burn 1988, Scott and Rose 1996, Snow and Perrins 1998). The species can be crepuscular in the winter, and often feeds by night (Kear 2005b) by bottom-feeding and diving (Snow and Perrins 1998) (most foraging being done at depths of 1-3 m) (Johnsgard 1978). Habitat The species requires extensive areas of nutrient-rich open water less than 6 m deep (Scott and Rose 1996) that is uncluttered with floating vegetation (Snow and Perrins 1998) but has abundant submerged macrophytes (Madge and Burn 1988, Kear 2005b), surrounding emergent (Madge and Burn 1988) vegetation and/or animal food (e.g. Chironomid larvae) (Kear 2005b). Breeding In its breeding range the species inhabits base-rich (e.g. saline, brackish or soda) lakes (Kear 2005b), eutrophic freshwater lakes, well-vegetated freshwater or brackish (Johnsgard 1978) marshes with areas of open water, swamps (del Hoyo et al. 1992, Scott and Rose 1996) and slow-flowing rivers (Madge and Burn 1988, del Hoyo et al. 1992, Scott and Rose 1996). Although it shows a strong preference for inland wetlands (Snow and Perrins 1998), the species will shift to coastal habitats such as sheltered coastal bays (Kear 2005b) when driven by frost or other compelling factors (Snow and Perrins 1998). Non-breeding During the winter the species frequents similar habitats to those it breeds in, including large lakes (Brown et al. 1982, del Hoyo et al. 1992, Scott and Rose 1996), slow-flowing rivers (Madge and Burn 1988, del Hoyo et al. 1992, Scott and Rose 1996), reservoirs (Brown et al. 1982, Madge and Burn 1988, del Hoyo et al. 1992, Scott and Rose 1996), brackish waters, marshes, weirs (Africa) (Brown et al. 1982) and flooded gravel pits (Fox et al. 1994). As in the breeding season, the species will shift to coastal habitats such as brackish lagoons (del Hoyo et al. 1992, Scott and Rose 1996), tidal estuaries (Madge and Burn 1988, del Hoyo et al. 1992, Scott and Rose 1996) and inshore waters (Madge and Burn 1988, Scott and Rose 1996) (where it associates with sewage outfalls) (Kear 2005b) when driven by frost or other compelling factors (Snow and Perrins 1998). Diet The species is omnivorous, its diet consisting of seeds (del Hoyo et al. 1992, Kear 2005b), roots (del Hoyo et al. 1992), rhizomes (Kear 2005b) and the vegetative parts of grasses, sedges and aquatic plants (Johnsgard 1978, del Hoyo et al. 1992, Kear 2005b), as well as aquatic insects and larvae (del Hoyo et al. 1992) (e.g. midge and caddis fly larvae during the summer) (Johnsgard 1978), molluscs, crustaceans, worms (del Hoyo et al. 1992)(oligochaetes) (Marsden and Bellamy 2000), amphibians (del Hoyo et al. 1992) (e.g. frogs and tadpoles) (Brown et al. 1982) and small fish (del Hoyo et al. 1992). Breeding site The nest is a depression (del Hoyo et al. 1992) or shallow cup (Kear 2005b) in a thick heap of vegetation (del Hoyo et al. 1992) positioned on the ground (Madge and Burn 1988, del Hoyo et al. 1992) (usually within 10 m of water) (Snow and Perrins 1998), in shallow water (Madge and Burn 1988, del Hoyo et al. 1992) (c.30 cm deep) (Johnsgard 1978), concealed in thick waterside vegetation (Madge and Burn 1988, del Hoyo et al. 1992) (e.g. reedbeds) (Johnsgard 1978), or on floating mats of reeds of other vegetation (Johnsgard 1978). 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 hummocks (Johnsgard 1978). 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) (Albrecht et al. 2006). 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) (Marsden and Bellamy 2000). 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 macrophytes (Giles 1994). The removed fish (dead or alive) were sold to generate funds (Giles 1994).
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Decreasing
