Justification of Red List Category
This species has an extremely large range, and hence does not approach the thresholds for Vulnerable under the range size criterion (Extent of Occurrence <20,000 km2 combined with a declining or fluctuating range size, habitat extent/quality, or population size and a small number of locations or severe fragmentation). The population trend appears to be stable, and hence the species does not approach the thresholds for Vulnerable under the population trend criterion (>30% decline over ten years or three generations). The population size is very large, and hence does not approach the thresholds for Vulnerable under the population size criterion (<10,000 mature individuals with a continuing decline estimated to be >10% in ten years or three generations, or with a specified population structure). For these reasons the species is evaluated as Least Concern.
The global population is estimated to number c.495,000-605,000 individuals (Wetlands International 2016). The European population is estimated at 70,100-120,000 pairs, which equates to 140,000-240,000 mature individuals (BirdLife International 2015).
The overall population trend is stable, although some populations have unknown trends (Wetlands International 2016). This species has had stable population trends over the last 40 years in North America (data from Breeding Bird Survey and/or Christmas Bird Count: Butcher and Niven 2007). In Europe the population size is estimated to be decreasing at a rate approaching 30% in 21.9 years (three generations) (BirdLife International 2015).
This species breeds in most of the northern North America, south to the Great Lakes, in Greenland (to Denmark), Iceland, and much of northern Eurasia south to the United Kingdom, parts of Eastern Europe, north-east China and northern Japan. Its wintering grounds expand its range to include the Atlantic and Pacific coasts of North America, areas of central Europe and the Mediterranean basement, the Black Sea, the southern Caspian Sea, the southern coast of Iran and Pakistan, the eastern coast of China, and the coasts of Korea and Japan (del Hoyo et al. 1992).
Behaviour This species is fully migratory (Madge and Burn 1988, del Hoyo et al. 1992) although in temperate regions it only undertakes short distance movements to nearby coasts (Scott and Rose 1996), or remains close to its breeding waters throughout the year (Madge and Burn 1988). It breeds from April or May (later in northern populations) (Kear 2005), in single pairs or colonies (del Hoyo et al. 1992), on islands or small islets, adults often assembling in groups on beaches when not at the nest (even when nesting solitarily) (Kear 2005). Males leave the breeding grounds in June to moult in small groups along the coast (Scott and Rose 1996), often considerable distances from the breeding areas (Snow and Perrins 1998) (although there is no evidence for any major migrations to common moulting sites) (Johnsgard 1978). The autumn migration begins in September (Scott and Rose 1996) and the species returns from the wintering grounds as early as February (Scott and Rose 1996). It is gregarious during the winter and on migration (Kear 2005), flocks of up to a hundred or more occurring in suitable sites during the autumn (although it travels in much smaller flocks during the spring) (Snow and Perrins 1998). Habitat Breeding The species breeds along the wooded shorelines (Kear 2005) of deep lakes (del Hoyo et al. 1992), small rivers and streams (del Hoyo et al. 1992) with moderate currents (Snow and Perrins 1998) in the tundra, boreal and temperate forest zones (Snow and Perrins 1998, Kear 2005), as well as on more saline waters such as sheltered shallow bays, inlets, straits or estuaries with sandy rather than muddy substrates (Snow and Perrins 1998). It shows a preference for narrow channels rather than open expanses of water, with islands or islets and spits, projecting rocks or grassy banks (Snow and Perrins 1998). Non-breeding The majority of the species winters at sea (del Hoyo et al. 1992), frequenting both inshore and offshore waters, estuaries, bays and brackish lagoons (del Hoyo et al. 1992), but showing a preference for clear, shallow waters not affected by heavy wave action (Johnsgard 1978). It will also utilise large freshwater lakes on passage (Madge and Burn 1988). Diet Its diet consists predominantly of small, shoaling marine or freshwater fish (del Hoyo et al. 1992), as well as small amounts of plant material (del Hoyo et al. 1992) and aquatic invertebrates (del Hoyo et al. 1992), such as crustaceans (e.g. shrimps and crayfish) (Johnsgard 1978), worms and insects (Kear 2005). Breeding site The nest is constructed within 25 m of water (Kear 2005) in a variety of locations such as natural cavities on the ground (Johnsgard 1978, del Hoyo et al. 1992, Kear 2005), burrows (del Hoyo et al. 1992, Kear 2005), under boulders (Madge and Burn 1988), amongst tree or scrub roots (Snow and Perrins 1998), in tree cavities, artificial nestboxes (Madge and Burn 1988), amongst reeds, or on floating reed mats (Flint et al. 1984). Where tree cavities or artificial nestboxes are utilised, the species shows a preference for those with entrances c.10 cm in diameter and with internal diameters of 30-40 cm (Johnsgard 1978). Management information The breeding density of this species increased on islands in the outer archipelago of south-west Finland as a result of feral American mink Neovison vison removal (Nordstrom et al. 2002). The species will also nest in artificial nestboxes with entrances c.10 cm in diameter and with internal diameters of 30-40 cm (Johnsgard 1978).
The species is subject to persecution and may be shot by anglers and fish-farmers who perceive it as a competitor and accuse it of depleting their fish stocks (del Hoyo et al. 1992, Kear 2005). The level of impact from direct persecution is currently unknown (Craik et al. 2015). The species is also hunted in North America (Kear 2005) and Denmark (Bregnballe et al. 2006), although it appears to be a less popular game species (Kear 2005). Eggs used to be, and possibly still are, harvested in Iceland (Gudmundsson 1979).
It may also threatened by accidental entanglement and drowning in fishing nets (Kear 2005). Alterations to its breeding habitats by dam construction and deforestation, and habitat degradation from water pollution could represent threats (del Hoyo et al. 1992). The species is susceptible to avian influenza and may be threatened by future outbreaks of the virus (Melville and Shortridge 2006). However, all of these threats may not have significant impacts on the species overall at the moment.
Conservation Actions Underway
CMS Appendix II. EU Birds Directive Annex II. The breeding density of this species increased on islands in the outer archipelago of south-west Finland as a result of feral American Mink (Neovison vison) removal (Nordstrom et al. 2002).
Conservation Actions Proposed
The erection of nest boxes may encourage the use of local areas by this species. Strict legislation on petroleum drilling and transport should be enforced and measures to reduce bycatch implemented and enforced. Important areas should be protected from drainage and other habitat modifications. Conduct research into the impact this species has on fish stocks and ways to minimise conflict should be established.
Text account compilers
Martin, R., Stuart, A., Butchart, S., Ashpole, J, Calvert, R., Ekstrom, J., Fjagesund, T., Hermes, C., Bennett, S., Malpas, L.
BirdLife International (2020) Species factsheet: Mergus serrator. Downloaded from http://www.birdlife.org on 09/08/2020. Recommended citation for factsheets for more than one species: BirdLife International (2020) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 09/08/2020.