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). Despite the fact that the population trend appears to 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 has not been quantified, but it is not believed to 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.
There is apparently no reliable estimate of this species's total population, and further research is needed.
The overall population trend is thought to be negative (Delany and Scott 2006), although there are mixed results from combined scoter Melanitta spp. surveys (T. Bowman in litt. 2012). According to one analysis, this species appears to have undergone a small or statistically insignificant increase over the last 40 years in North America (data from Breeding Bird Survey and/or Christmas Bird Count [CBC] [Butcher and Niven 2007]). Note, however, that these surveys cover less than 50% of the species's range in North America (Butcher and Niven 2007), and CBC data may not be reliable for tracking trends in this species, as it winters predominantly offshore, where a large proportion may not be visible to shore-based observers (T. Bowman in litt. 2012).
Behaviour This species is highly migratory (Madge and Burn 1988, del Hoyo et al. 1992) and breeds from mid-May onwards (Madge and Burn 1988) in solitary pairs or loose groups (del Hoyo et al. 1992, Kear 2005), with neighbouring nests as close as 3 m apart (Snow and Perrins 1998). It occasionally nests in association with gull or tern colonies (Kear 2005). Non-breeding birds spend the breeding season in flocks on open water (Flint et al. 1984). After breeding (from June onwards [Scott and Rose 1996]) the adults migrate to moulting sites (males travelling and moulting before the females) (Madge and Burn 1988), where they become flightless for 3-4 weeks (Scott and Rose 1996). When moulting and overwintering the species is highly sociable and can occur in large flocks (Madge and Burn 1988, Scott and Rose 1996) of several thousands of individuals (Scott and Rose 1996), although it is more common in small scattered groups of c.100 individuals (Snow and Perrins 1998). The species mainly forages by diving and may feed at depths of 30-40 m during the winter (del Hoyo et al. 1992). Habitat Breeding The species breeds on wooded coastlines (Johnsgard 1978, Kear 2005), small freshwater lakes (del Hoyo et al. 1992, Snow and Perrins 1998, Kear 2005), pools and rivers (Snow and Perrins 1998) in northern coniferous forests (Johnsgard 1978, del Hoyo et al. 1992, Snow and Perrins 1998, Kear 2005), wooded Arctic tundra (del Hoyo et al. 1992, Snow and Perrins 1998) and alpine zones (Snow and Perrins 1998, Kear 2005); especially where there are boulder-covered or small rocky islands available for nesting with extensive herbaceous vegetation, shrubs and low trees (Johnsgard 1978, Kear 2005). Non-breeding The majority winter at sea on shallow inshore coastal waters (Madge and Burn 1988, del Hoyo et al. 1992), especially in estuaries or inlets where there are large mussel-beds (Snow and Perrins 1998). The species may also occur on freshwater lakes and estuaries during migration (Madge and Burn 1988, Kear 2005). Diet Its diet consists predominantly of molluscs, as well as crustaceans, worms, echinoderms (del Hoyo et al. 1992), amphipods, isopods (Kear 2005), small fish, and (in freshwater habitats) adult and larval insects (del Hoyo et al. 1992). The species may also consume plant material on its breeding grounds (del Hoyo et al. 1992) (e.g. leaves and shoots) (Flint et al. 1984). Breeding site The nest is a shallow depression positioned on the ground (del Hoyo et al. 1992) in tall grass, among hummocks or under bushes (Flint et al. 1984), usually within 100 m of open water (occasionally up to 2-3 km away) (Kear 2005).
Hunting poses a significant threat to the species, with an estimated harvest of 6,000 – 14,000 individuals in the 10-year period from 2004-2014 in US and Canada, but with a downward trend (Canadian Wildlife Service Waterfowl Committee 2015). A recent estimate suggests that the harvest of White-winged Scoters may be 29-37% over the maximum sustainable rate (Koneff et al. 2017).
The species feeds at depths of 30–40 m and is therefore highly susceptible to bycatch in fishing nets, as documented by Good et al. (2009) in the Salient Sea, Washington. Moulting and wintering populations are also vulnerable to oil spills as they often occur in high densities in close proximity to oil transportation routes. One catastrophic spill could impact a large proportion of the population. Oil pollution can also cause the long-term disruption of food supplies which may have more serious effects on populations in the long-term than direct mortality from oiling. Lance (2001) documented evidence for a long-term negative effect as a result of the 1989 Exxon Valdez accident. The disturbance and habitat degradation caused by increasing levels of oil and gas exploration in the Arctic will likely have only minor or negligible impacts on this species (Poland et al. 2003, Henderson and Loe 2014).
In some locations in the Pacific Northwest, sea ducks could be exposed to toxicologically significant levels of cadmium associated with mussels foraged from aquaculture structures, which raises the possibility that such exposure could be contributing to observed population declines (Bendell 2011). Climate change is an additional threat, with remote sensing showing the shrinkage of ponds in subarctic Alaska (Riordan et al. 2006), with these wetlands providing important breeding grounds for the species at present. In general, climate change is predicted to cause dramatic habitat changes in the Arctic region (Fox et al. 2015). Decreasing spring snow cover duration in boreal regions has been linked to population declines of Scoters, likely due to trophic mismatch, projected to cause a 31.0% population decline between 1980 and 2080 (Drever et al. 2011). Ocean acidification may lead to declines in molluscs which form a large part of White-winged Scoter diet (Steinacher et al. 2009, del Hoyo et al. 2017).
Text account compilers
Taylor, J., Arendarczyk, B., Ekstrom, J., Fjagesund, T., Butchart, S., Hermes, C., Malpas, L., Palmer-Newton, A., Stuart, A.
Bowman, T., Pihl, S.
BirdLife International (2023) Species factsheet: Melanitta deglandi. Downloaded from http://datazone.birdlife.org/species/factsheet/white-winged-scoter-melanitta-deglandi on 26/09/2023. Recommended citation for factsheets for more than one species: BirdLife International (2023) IUCN Red List for birds. Downloaded from http://datazone.birdlife.org on 26/09/2023.