NT
Common Eider Somateria mollissima



Justification

Justification of Red List Category
This species has been uplisted to Near Threatened. Within Europe, it has experienced moderate declines which have not been compensated for by increases elsewhere in the species' range. Declines are thought to be driven by a range of threats including overharvesting of aquatic resources, pollution, disturbance and hunting. Should the declines be found to be more severe, or new information reveal declines in the S. m. sedentaria and S. m. borealis populations, then the species would warrant uplisting; it almost meets the requirements for listing as threatened under criterion A4abcde.

Population justification

The global population is estimated to number c. 3,300,000-4,000,000 individuals (Wetlands International 2012), which equates to 1,580,000-1,910,000 mature individuals (BirdLife International 2015). The European population is estimated at 791,000-955,000 pairs.

Trend justification
In Europe the population size is currently declining overall at a rate of >40% over three generations (27 years) (BirdLife International 2015). A decline has been evident since the late 1990s in the largest flyway population of S. m. mollissima in the Baltic and Wadden Seas, based both on breeding data (Ekroos et al. 2012) and on midwinter counts conducted as part of the International Waterbird Census (Nagy et al. 2014). Rapid declines have been reported for the islands of Gotland (from 7,140 nesting females in 2007 to 1,310 in 2015) and Öland in the Baltic Sea (K. Larsson in litt. 2015). Europe (including Greenland) holds >60% of the global population (Wetlands International 2012), so the declines in Europe are globally significant.

The remainder of the population occurs in North America where population trends are variable. The Pacific population, S. m. v-nigra, which represents c. 4% of the global population is thought to have declined in the northern parts of its range between the 1980s and early 2000s,  while in central Arctic Canada and north-west Alaska it has declined, however, it appears to be increasing in the rest of Alaska (Bowman et al. 2015). The American population, S. m. dresseri, (c. 9% of the global population) shows variable trends with the northern population increasing and southern population decreasing. Trends are uncertain for the Hudson Bay, S. m. sedentaria, (c. 6% of the global population) and Northern, S. m. borealis, (c. 16% of the global population) populations. Given the strong declines in the European population and a lack of compensatory increases in the North American population the overall population trend is thought to be declining moderately rapidly.

Distribution and population

The breeding range of this species is widely distributed around the mid- to high-latitude coasts of the northern hemisphere, particularly at the higher latitudes. It has a Holarctic distribution, being present in both Eurasia and North America. However, its distribution is not continuous across the Holarctic. There are distinct and persistent gaps in both the central parts of the Russian Arctic and the Canadian Arctic. The distributional gap in Russia (54° longitude) is considerably wider than that in Canada (6° longitude). These gaps can be attributed to the presence of ice all year round at these locations, especially landfast sea ice. At the highest latitudes, the presence of breeding birds is largely limited by the distribution of ice-free conditions during June.

In winter, the range is more compressed, with migratory northern populations largely wintering within the range of the southerly sedentary populations (Waltho and Coulson 2015). The southern limits of regular wintering ranges are southern Alaska (USA) at 51°N, New Jersey (USA) at 41°N, W France at 44°N and Kuril Islands (Russia) at 50°N (Waltho and Coulson 2015).

Globally there are six widely recognised subspecies (del Hoyo et al. 1992, Waltho and Coulson 2015), with four in Eurasia: European (S. mollissima mollissima), Faroes (S. m. faeroeensis), Northern (S. m. borealis) and Pacific (S. mollissima v-nigrum). There are four subspecies in North America: Pacific (S. mollissima v-nigra), American (S. m. dresseri), Hudson Bay (S. m. sedentaria), and Northern (S. m. borealis) (Bowman et al. 2015).


Ecology

The Common Eider always nests on the ground, and usually in areas free of mammalian predators (Waltho and Coulson 2015), including coastal islands and islets along low-lying rocky coasts, on coastal shores and spits, on islets in brackish and freshwater lagoons, coastal lakes and rivers or on tundra pools close to the coast (Johnsgard 1978; del Hoyo et al. 1992; Kear 2005). They use a wide range of nesting habitats, including bare ground, shingle, grassland, scrub, under trees and in derelict buildings (Waltho and Coulson 2015). The nest is a slight hollow in the ground that is usually positioned in the shelter of rocks or vegetation but may also be in the open (del Hoyo et al. 1992). Large nesting aggregations form where safe nesting sites are at a premium (Waltho and Coulson 2015).

These species typically moults on shallow marine or sheltered coastal waters free from disturbance (Kear 2005; Waltho and Coulson 2015). It winters on shallow coasts (usually <10m depth), bays and estuaries (del Hoyo et al. 1992; Waltho and Coulson 2015), especially where there are high abundances of benthic molluscs, particularly Mytilus edulis (Camphuysen et al. 2002, Ens 2006, Waltho and Coulson 2015). It may also occur inland on freshwater lakes when on passage and during the winter (rarely) (Waltho and Coulson 2015). Its diet consists predominantly of benthic molluscs, especially Mytilus edulis, although a wide range of crustaceans (e.g. amphipods and isopods), echinoderms, other marine invertebrates and fish may also be taken (Johnsgard 1978, del Hoyo et al. 1992, Waltho and Coulson 2015). During the breeding season incubating females in the Arctic  are reported to feed on  algae, berries and the seeds and leaves of surrounding tundra plants (del Hoyo et al. 1992), while in temperate regions they do not feed, which leads to the temporary breakdown of their digestive system (Waltho and Coulson 2015, Laursen and Møller 2016), Northern populations of this species are migratory (>1000km), but southern populations are largely sedentary (Waltho and Coulson 2015). Migratory nature is largely a response to coastal sea ice (Waltho and Coulson 2015).

Threats

The critical factor for the long-term survival of the species appears to be the survival rate of young ducklings (Waltho and Coulson 2015), but it is not clear which factors are causing the low duckling survival. Past mass mortality in ducklings has been caused by viral infections (1996, 1999) (Hollmén 2002), and the impact of this is likely aggravated by low availability and low nutritional quality of food supplies which is associated with poor body condition in females at the onset of the breeding season (Desholm et al. 2003). Avian cholera appears to be a potentially significant threat with mass mortality events noted at several colonies in Denmark and in Canada (Kats 2007, Descamps et al. 2012, Tjørnløv et al. 2013), and according to Descamps et al.’s (2012) model, only a few recurrent outbreaks would be sufficient to cause colony extinction within a relatively short time. Other diseases may also impact populations, e.g. Wellfleet Bay Virus has caused heavy mortality in Massachusetts, US (Ballard et al. 2012) but high parasite loads are considered to be a symptom of other detrimental impacts rather than a primary driver of decline (Kats 2007).

Conflict with the shellfish aquaculture industry can have detrimental effects on certain populations (Kear 2005, Ens 2006). Over-harvesting of benthic molluscs has led to mass starvation events in the past, particularly where secondary prey resources are targeted, as with intensive fishing of Spisula clams during a period of low mussel stocks prior to a mass mortality event in 1999-2000 in the Dutch Wadden Sea and southern North Sea (Camphuysen et al. 2002, Ens 2006, Kats 2007). The species is also impacted by the extensive use of gillnets, and commonly becomes entangled and drowned in monofilament nets, and significant mortality may occur where fishing effort overlaps with Eider aggregations (Kear 2005, Žydelis et al. 2013).

Levels of hunting are high within several countries within the range, and have been considered unsustainable in Russia (Nikolaeva et al. 2006), Denmark (Bregnballe et al. 2006) and Greenland (where annual bags exceeded 80,000 in the late 1990s) (Merkel 2004). New restrictions introduced in 2004 appear to have enabled populations to recover considerably (Merkel 2010). In North America, the harvest may still exceed sustainable levels in certain areas (Kear 2005).  Populations in the high Arctic are subject to shooting by indigenous peoples for food, especially in spring (Byers and Dickson 2001, Kear 2005), but this subsistence hunting is likely to be sustainable at current levels (Byers and Dickson 2001). An associated threat is that of lethal and sub-lethal lead exposure through ingestion of lead shot by adult birds, with 23% of females dying of emancipation after incubation were diagnosed with severe or sub-clinical lead poisoning (Hollmén 2002), and 22% of birds collected through hunting in Greenland over three years carried embedded shot pellets (Falk et al. 2006).

The species is vulnerable to chronic coastal oil pollution and to major oil spills especially in areas where large moulting and wintering concentrations occur (Kear 2005, Nikolaeva et al. 2006, Waltho and Coulson 2015, Carboneras et al. 2017). While one major incident has the potential to be disastrous, levels of chronic oil-related mortality appears relatively stable in the southern North Sea and Wadden Sea, at between 4-12% of estimated winter mortality (Camphuysen et al. 2002).

Disturbance from the development of mineral resources along the coast (Nikolaeva et al. 2006) and from local shore-based activities such as angling and dog walking increases the likelihood of predation on young (Keller 1991). Other recreational activities such as boat tours (Burnham et al. 2012), as well as scientific research (Bolduc and Guillemette 2003) are also likely to reduce productivity at certain locations (Keller 1991). In winter, unregulated tourism and shipping also cause disturbance to the species, and may prevent access to favoured feeding grounds (Nikolaeva et al. 2006).

Additional threats include predation of breeding females in Europe by the increased population of White-tailed Eagle Haliaeetus albicilla and nest predation by introduced mink Mustela vison (Kats 2007, Ekroos et al. 2012) and in the Americas by introduced foxes Vulpes spp. (Petersen et al. 2015). Increasing incidence of nesting in tight association with gull colonies and in less exposed sites, may reflect antipredator responses.

The species may also be sensitive to the effects of climate change, though assessing the impacts on population dynamics is problematic as negative effects (e.g. lower growth in prey leading to reduced food availability [Carboneras et al. 2012]) are confounded by potential positive impacts deriving from earlier arrival on breeding grounds improving breeding success in some areas (D’Alba et al. 2010).

Mussel quality, and subsequent eider breeding condition, increases with higher nutrient content of coastal waters (Laursen and Møller 2014) so recent reductions in the level of eutrophication of near-shore waters may have contributed to lower food availability.  Thiamine deficiency has been linked to a lethal paralytic syndrome that has caused mass mortality of Eider ducklings. Unlike Herring Gulls Larus argentatus, Common Eider females will lay eggs even below the critical threshold of thiamine, with resultant rapid mortality in hatchlings (Balk et al. 2009). The extent to which this impacts the population is unknown, and is unlikely to account for the strongly skewed sex ratio observed in the species (Lehikoinen et al. 2008). However, the causes of lethal thiamine deficiency need further investigation as there may be a widespread pollutant present that is currently unidentified. Sand and gravel extraction destroys habitats of benthic organisms, with potential reverberations up the trophic chain. In the Baltic, the impact is thought to be sufficiently low to not cause declines (Skov et al. 2011).

Conservation actions

Conservation and Research Actions Underway
EU Birds Directive Annex II and III. CMS Appendix II. Changes to hunting regulations in Greenland in 2001 shortened the length of the hunting season which is thought to have led to a rapid increase in population size (Burnham et al. 2012). However the hunting regulations have recently changed and the effect on the population is not yet known. Restrictions were also introduced in Denmark in 2004/2005 and 2011/2012 with the aim of reducing the proportion of female birds killed and increasing the population growth rate (Christensen and Hounisen 2014).

Conservation and Research Actions Proposed
Sustainable levels of hunting should be established in those areas where the species is harvested and legislation established and enforced to ensure this. Conduct flyway-scale winter counts (Ekroos et al. 2012) and ensure monitoring is coordinated across the species's range (I. K. Petersen in litt. 2015). Key areas should be protected from all forms of disturbance including oil exploration, drilling and transportation. Further research is needed into impacts of declining nutrient levels in coastal waters and their effects on mussel populations, and the subsequent effects of this on site carrying capacity, body condition and subsequent breeding productivity, across the species range. Levels of shellfish harvesting should be monitored to ensure sustainability and measures to minimise bycatch in fishing nets promoted amongst fisheries. International monitoring plans should be developed and a programme of research put in place, while taking precautions to minimise the impact of scientific work.

Identification

50-71 cm. Breeding male very distinctive with black coronal region, divided by white streak over central crown to nape (Carboneras et al. 2014). Rest of head white with pale green patches on nape and rear-ear coverts. Upperparts white, breast rosy-pink, rump, tail and rest of underparts black. Bill olive-grey with pale yellow tip. Female warm brown with black bars. Juveniles resemble female. Voice Characteristic cooing call between autumn and spring, otherwise generally silent.

Acknowledgements

Text account compilers
Malpas, L., Martin, R., Moreno, R., Palmer-Newton, A., Pople, R., Stuart, A., Wheatley, H., Wright, L, Ashpole, J, Butchart, S., Burfield, I., Calvert, R., Ekstrom, J., Fjagesund, T., Hermes, C., Ieronymidou, C.

Contributors
Pihl, S., Gudmundsson, G., Meltofte, H., Larsson, K., Waltho, C., Drysdale, A., Petersen, I., Coulson, J., Laursen, K.


Recommended citation
BirdLife International (2019) Species factsheet: Somateria mollissima. Downloaded from http://www.birdlife.org on 23/10/2019. Recommended citation for factsheets for more than one species: BirdLife International (2019) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 23/10/2019.