Behaviour This species is highly migratory (del Hoyo et al. 1992), often resting and feeding on inland waterbodies when on passage (Kear 2005). It arrives on the breeding grounds from April (Kear 2005) or early-May and breeds from mid-May onwards (Madge and Burn 1988) in single pairs or loose groups (del Hoyo et al. 1992). Males gather in large flocks close to the breeding grounds after mating to undergo a flightless moulting period (Madge and Burn 1988). The species then leaves the breeding grounds in early-September and October (Madge and Burn 1988, Kear 2005). Outside of the breeding season the species is highly gregarious and occurs in small or large flocks (Kear 2005) usually not exceeding 100 individuals, although larger gatherings may form at major passage waters during migration (Madge and Burn 1988) and flocks of over 10,000 have been recorded during the winter (Kear 2005). The species feeds diurnally (Madge and Burn 1988) by diving to depths of 1-4 m (Kear 2005) and forms gregarious nocturnal roosts (Kear 2005). Habitat Breeding It breeds on freshwater oligotrophic lakes, pools (del Hoyo et al. 1992, Kear 2005), oxbow lakes (Kear 2005b), backwaters of large slow-flowing rivers (del Hoyo et al. 1992, Snow and Perrins 1998, Kear 2005), muskegs (bogs) (del Hoyo et al. 1992, Kear 2005) and flooded riverside woods (Johnsgard 1978, Snow and Perrins 1998) in the coniferous and mixed deciduous/evergreen forest zones (del Hoyo et al. 1992, Snow and Perrins 1998). It shows a preference for shallow water (Johnsgard 1978) up to 4 m (Kear 2005) (maximum 6 m) deep (Snow and Perrins 1998), and requires mature broadleaved trees (e.g. oaks, willows and aspens) with holes in which to nest (Johnsgard 1978). Non-breeding The species overwinters on large freshwater lakes (del Hoyo et al. 1992) and reservoirs (Johnsgard 1978), ice-free rivers (Kear 2005), brackish coastal lagoons, estuaries (del Hoyo et al. 1992) and sheltered coastal bays (Madge and Burn 1988) (although rarely on the open sea) (del Hoyo et al. 1992), often resting and feeding on small bodies of water or small streams when on passage (Kear 2005b). Diet During the breeding season its diet consists predominantly of benthic aquatic invertebrates such as adult and larval insects (del Hoyo et al. 1992, Kear 2005), crustaceans, molluscs and polycheate worms (Kear 2005b), as well as amphibians (del Hoyo et al. 1992), fish (Kear 2005) and plant matter (del Hoyo et al. 1992) (seeds, leaves and roots) (Kear 2005b). During the winter and in early spring however the species mainly feeds on fish (del Hoyo et al. 1992). Breeding site The species nests in tree hollows (del Hoyo et al. 1992) up to 10 m or more above the ground (Kear 2005) (especially those excavated by Black Woodpeckers Dryocopus martius) (Madge and Burn 1988) in mature broadleaved trees (e.g. oak, willow or aspen) (Johnsgard 1978). It may also nest in artificial nest boxes (del Hoyo et al. 1992, Kear 2005), especially those erected to attract Common Goldeneye Bucephala clangula (Madge and Burn 1988, Snow and Perrins 1998).

Conservation Actions Underway
CMS Appendix II. Bern Convention Appendix II. EU Birds Directive Annex I. The following information refers to the species's European range only: The total population of this species wintering in north-east Europe has increased from 6% to a third in two decades. SPAs have proven effective in accommodating this range shift, with population growth in this region doubling inside EU Birds Directive's SPAs compared to those outside over the last 25 years (Pavón-Jordán et al. 2015).

Conservation Actions Proposed
The following information refers to the species's European range only: Gaps in the current network of protected areas need to be filled and national and international assessments of the EU Natura 2000 network should be made regularly (Pavón-Jordán et al. 2015). Strict legislation on oil transportation needs to be implemented and enforced to minimise the risk of spills. Investigation into the impacts of American Mink should be undertaken and suitable eradication methods trialled. In areas where the species is hunted, research and legislation is needed to ensure that it is sustainable.