Habitat and Ecology
Habitat and Ecology
Behaviour Northern populations of this species are highly migratory3, 7, with those breeding in the milder parts of western or southern Europe7 being sedentary1, 3, 7 or only making short-distance dispersal movements1, 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 individuals3, 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 macrophytes2, 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, swamps1, 3 and slow-flowing rivers1, 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 lakes1, 3, 6, slow-flowing rivers1, 3, 4, reservoirs1, 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 lagoons1, 3, tidal estuaries1, 3, 4 and inshore waters3, 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 seeds1, 2, roots1, rhizomes2 and the vegetative parts of grasses, sedges and aquatic plants1, 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 ground1, 4 (usually within 10 m of water)7, in shallow water1, 4 (c.30 cm deep)5, concealed in thick waterside vegetation1, 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.