Smew Mergellus albellus


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 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.

Population justification
The global population is estimated to number > c.130,000 individuals (Wetlands International 2015). The European population is estimated at 9,200-17,600 pairs, which equates to 18,300-35,200 mature individuals (BirdLife International 2015).

Trend justification
The overall population trend is suspected to be decreasing however some populations are increasing or the trend is not known (Wetlands International 2015). The population is suspected to be much affected by availability of nest sites in suitable habitat. Populations are also subject to hunting pressure during migration and have been affected by pollution from oil spills (del Hoyo et al. 1992). In Europe the population size is estimated to be increasing (BirdLife International 2015).


Behaviour This species is highly migratory (del Hoyo et al. 1992), often resting and feeding on inland waterbodies when on passage (Kear 2005b). It arrives on the breeding grounds from April (Kear 2005b) 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 2005b). Outside of the breeding season the species is highly gregarious and occurs in small or large flocks (Kear 2005b) 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 2005b). The species feeds diurnally (Madge and Burn 1988) by diving to depths of 1-4 m (Kear 2005b) and forms gregarious nocturnal roosts (Kear 2005b). Habitat Breeding It breeds on freshwater oligotrophic lakes, pools (del Hoyo et al. 1992, Kear 2005b), oxbow lakes (Kear 2005b), backwaters of large slow-flowing rivers (del Hoyo et al. 1992, Snow and Perrins 1998, Kear 2005b), muskegs (bogs) (del Hoyo et al. 1992, Kear 2005b) 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 2005b) (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 2005b), 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 2005b), crustaceans, molluscs and polycheate worms (Kear 2005b), as well as amphibians (del Hoyo et al. 1992), fish (Kear 2005b) 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 2005b) (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 2005b), especially those erected to attract Common Goldeneye Bucephala clangula (Madge and Burn 1988, Snow and Perrins 1998).


Where it occurs in large numbers on coastal waters the species is particularly vulnerable to oil pollution (del Hoyo et al. 1992). Populations declined in Europe throughout the 19th and 20th centuries due to habitat degradation and loss (e.g. the loss of mature trees in river valleys as a result of logging, conversion to agriculture and river canalisation) (Kear 2005b). The species has also suffered local declines as a result of predation by American mink Neovison vison (Kear 2005b), and is susceptible to avian influenza so may be threatened by future outbreaks of the virus (Melville and Shortridge 2006). Climate change is also affecting the range of this species (Pavón-Jordán et al. 2015). Utilisation The species is susceptible to a certain amount of hunting pressure when on passage and during the winter (del Hoyo et al. 1992).

Conservation actions

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.


Text account compilers
Ekstrom, J., Butchart, S., Malpas, L., Symes, A., Ashpole, J

Pihl, S.

Recommended citation
BirdLife International (2023) Species factsheet: Mergellus albellus. Downloaded from on 02/02/2023. Recommended citation for factsheets for more than one species: BirdLife International (2023) IUCN Red List for birds. Downloaded from on 02/02/2023.