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). Although the population may be decreasing, the decline is not believed to be sufficiently rapid to approach the thresholds for Vulnerable under the population trend criterion (>30% decline over ten years or three generations). The population size is extremely 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. 4,920,000-5,130,000 individuals (Wetlands International 2016). The breeding population in Europe is estimated at 134,000-178,000 pairs, which equates to 269,000-355,000 mature individuals (BirdLife International 2015).
The overall population trend is decreasing, although some populations may be stable or have unknown trends (Wetlands International 2016). This species underwent a significant population increase between 1974 and 2011 in North America (Zimpfer 2011, T. McCoy in litt. 2012). In Europe the breeding population size is estimated to be decreasing by less than 25% in 24.6 years (three grenerations) (BirdLife International 2015).
Summer breeding grounds of the species range across the northern limits of Europe (including Iceland) and Asia, through the Aleutian Islands (year-round breeding) to Alaska (USA), and across to the Atlantic coast of Canada (del Hoyo et al. 1992). It winters further south, reaching California, the great lakes and northern Florida in North America, the Adriatic Sea and northern Black Sea in Europe, the western Caspian Sea, and on the Pacific coast of Asia as far as south-east China (del Hoyo et al. 1992).
Behaviour This species is fully migratory (del Hoyo et al. 1992). It breeds in the high Arctic from late-May or early-June (depending on the timing of the Arctic thaw) in single pairs or loose groups, often with colonies of nesting gulls or terns (del Hoyo et al. 1992; Kear 2005); although it is not itself a colonial species (Snow and Perrins 1998). Males undergo short post-breeding moult migrations, often gathering in small or large flocks (up to 4,000 in Iceland) (Madge and Burn 1988; Scott and Rose 1996) while females are incubating (Madge and Burn 1988). Females usually moult in the breeding grounds (Madge and Burn 1988, Scott and Rose 1996), although large concentrations (500-1,000 individuals) of moulting females have been recorded away from breeding areas (Scott and Rose 1996). During the moulting period the species is flightless for a period of c.3-4 weeks (Scott and Rose 1996). The autumn migration begins after the moulting period in mid-August (Scott and Rose 1996), with males tending to remain much further north than females or immatures (del Hoyo et al. 1992) leading to some sexual segregation during the winter (Madge and Burn 1988). The species is highly gregarious outside of the breeding season and is commonly observed in small or large flocks sometimes of several thousand individuals (Madge and Burn 1988; Scott and Rose 1996; Kear 2005). The return spring migration generally begins in late-February (Scott and Rose 1996) although flocks of non-breeders may remain in the south, often in winter quarters, during the breeding season (Madge and Burn 1988).
Habitat Breeding The species breeds in tundra, wooded tundra and moorland regions in the high Arctic, occupying small, shallow, freshwater lakes, pools and rivers with grassy shorelines and high densities of invertebrate life (Johnsgard 1978; del Hoyo et al. 1992; Scott and Rose 1996; Kear 2005). It shows a preference for water less than 6 m deep (usually 2 m) for diving (Kear 2005). Non-breeding The species winters on shallow coastal waters less than 10 m deep (especially in the vicinity of sewage outlets), as well as sheltered bays, estuaries and brackish coastal lagoons (del Hoyo et al. 1992; Scott and Rose 1996; Kear 2005). It is also found inland on large lakes and reservoirs during this season (Johnsgard 1978; Madge and Burn 1988; del Hoyo et al. 1992, Kear 2005).
Diet The species is omnivorous (Kear 2005). During the winter, its diet consisting predominantly of molluscs (Johnsgard 1978) such as muscles, cockles and clams while in coastal habitats and Hydrobia snails while in brackish waters (del Hoyo et al. 1992; Kear 2005). Other food sources include insects, aquatic insect larvae, crustaceans such as amphipods, worms, small fish, and the roots, seeds and vegetative parts of aquatic plants such as sedges and water weeds (Johnsgard 1978; del Hoyo et al. 1992; Kear 2005).
Breeding The nest is a shallow depression close to water on the ground, either in thick vegetation, in cracks in rocks, under woody shrubs or under perennial herbaceous vegetation less than 50 cm high (Iceland) (Johnsgard 1978; Madge and Burn 1988; del Hoyo et al. 1992; Kear 2005). The species is not colonial but it will sometimes nest among gulls and terns (Kear 2005), with neighbouring nests being placed as close as 1 m in some areas (Snow and Perrins 1998). In Scotland, UK the introduction of a sewage treatment scheme in the Firth of Forth, a large marine bay, resulted in a considerable reduction in the abundance of the species, with feeding flocks only remaining at outfalls where sewage continued to be discharged in large quantities (Campbell 1984). It was unclear whether the changes in the species's distribution were due to reductions in the number of food items borne in the sewage or to reductions in aquatic invertebrate abundance as a result of the new treatment system (Campbell 1984).
Incidental capture in commercial fisheries poses a major threat to this species which shows a relatively high vulnerability to bycatch (Sonntag et al. 2012). Entanglement in fishing equipment causes an estimated 5-10% annual mortality in the Baltic region (Jensen 2009). The species winters near major fishing areas in Europe, North America and Japan, thus interaction with fisheries is common throughout the range.
Wetland habitats critical to breeding success are potentially threatened by climate change. Remotely sensed imaging has revealed landscape-level changes, including the shrinkage of ponds in subarctic Alaska, associated with a warming climate, longer growing seasons and increased potential evapotranspiration (Riordan et al. 2006). Audubon’s (2014) climate model predicts the loss of over half of the North American summer range with suitable climatic conditions, with similar trends likely in Eurasia. Climate change is predicted to cause dramatic habitat changes throughout the sensitive ecosystems in the Arctic region (Fox et al. 2015). Reduced duration of spring snow cover in boreal regions has been linked to population declines of Greater Scaup, possibly due to trophic mismatch, with a projected 12.9% decline in population by 2080, compared to the 1980 census (Drever et al. 2011). Future ocean acidification may lead to declines in molluscs which form a large part of Scaup diet (Steinacher et al. 2009, Carboneras and Kirwan 2017). Certain colonies in the Pacific Northwest may be exposed to toxicologically significant levels of cadmium originating from mussels foraged from aquaculture structures, raising the possibility that heavy metal pollution could be contributory to observed population declines (Bendell 2011). The Scaup’s habit of congregating around coastal sewage outlets in winter, also exposes it to risk from other pollutants (Carboneras and Kirwan 2017).
Flocks often occur in vicinity of oil-related developments and are therefore susceptible to oiling and related pollutants, particularly when moulting and during winter. Oil pollution could also impact the species indirectly through decreased abundance or quality of invertebrate prey (Kirby et al. 1993, Kear 2005). Oil and gas exploration in Siberia may have disruptive impacts on breeding grounds and could potentially be associated with an increase in hunting pressures (Jensen 2009, Mineyev 1998). Wind farms may represent a threat to the Greater Scaup, having been shown to affect duck population densities more severely than those of other species groups (Stewart et al. 2007). However, it has been found that collision rates are lower than for other groups (Dürr 2013 per Fox et al. 2015) and, as such, wind farms are likely to only be damaging to the population when inappropriately placed (Drewitt & Langston 2006).
Conservation Actions Underway
CMS Appendix II. This species is listed on Annex II (and III) of the EU Birds Directive and can only be hunted in those ten Member States specifically mentioned in the Birds Directive: Belgium, Denmark, Germany, Greece, France, Ireland, Latvia, Netherlands, Romania and U.K.. There are management plans to reduce fishery activity at two sites in the Netherlands (Ijsselmeer and Waddenzee) and in Finland a large-scale eradication of Mink (Mustela vison) in the Quark as an EU LIFE project (Jensen 2009) began in 2001 (Anon 2005).
Conservation Actions Proposed
Preservation of large areas of breeding habitat in northern Europe and its highly localised north-west European wintering grounds. Identify Important Bird Areas and protect habitats and sites. Implement stricter regulations on oil exploitation and transportation. Restrict hunting. Conduct further research on the effects of disturbance from hunting and the nomadic habits of the species outside the breeding season (Tucker and Heath 1994). Continue Mink eradication programme.
Text account compilers
Malpas, L., Palmer-Newton, A., Stuart, A., Butchart, S., Arendarczyk, B., Calvert, R., Ashpole, J, Derhé, M., Ekstrom, J., Fjagesund, T., Hermes, C.
Ellermaa, M., Lehikoinen, A., DeVink, J., Szymanski, M., Barclay, J., Kontiokorpi, J., Bianki, V., McCoy, T., Burfield, I., Kharitonov, S., Lehikoinen, E., Kharitonova, I.
BirdLife International (2021) Species factsheet: Aythya marila. Downloaded from http://www.birdlife.org on 12/04/2021. Recommended citation for factsheets for more than one species: BirdLife International (2021) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 12/04/2021.