Justification 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 increasing, 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 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.

Family/Sub-family Anatidae

Species name author Linnaeus, 1758

Taxonomic source(s) AERC TAC (2003), AOU checklist (1998 + supplements), Cramp and Simmons (1977-1994), Sibley and Monroe (1990, 1993), Stotz et al. (1996)

Population estimate

Population trend

Range estimate (breeding/resident)

Country endemic?

1,700,000 - 2,400,000

unset

21,900,000 km²

No

Range & population The Common Merganser can be found breeding year-round in central Asia and western China, in northern Europe including France, Germany, Scandinavia, the United Kingdom and Iceland, and in large areas of northern United States and parts of southern Canada. Summer breeding grounds include the majority of Scandinavia and Russia, parts of central Asia including northern India, northern regions of Kazakhstan, Mongolia and Japan, and much of southern Canada. Wintering grounds expand the range further south to encompass most of the United States, other coastal regions of south-eastern Europe (e.g. Turkey and Greece) and central Asia, and the eastern coast of China as well as Korea and Japan¹.

Important Bird Areas Click here to view map showing IBAs where species is recorded and triggers any of the IBA criteria.

Ecology: Behaviour Northern breeding populations of this species are fully migratory⁶ although breeders in temperate regions are sedentary or only travel short distances^{1, 5}. The species arrives on its breeding areas between March and May⁵, actually breeding as early as late-March (although often considerably later in more northerly regions)³. It breeds in solitary pairs or loose groups¹ of up to 8-10 individuals⁷ (especially on islands that provide suitable nesting sites in lacustrine or coastal locations)⁴. The males gather to moult after mating, often undertaking considerable moult migrations to flock on key waters³. Females also undergo a post-breeding moult on the breeding grounds during which they become flightless for around 1 month⁴. The main migration to wintering areas largely occurs from October through to December⁵, mass departures being linked to the freezing of breeding and moulting areas⁶. Outside of the breeding season the species is typically found in small parties, with groups of up to 70 individuals feeding together on shoals of fish during the winter². Large flocks may also form on roosting waters³, the largest gatherings occurring during the autumn migration and winter months and numbering up to several thousands of individuals³ (1,000 to 10,000 on suitable waters)⁶. On passage in the spring flocks are usually small however, breeding pairs remaining in flocks until they reach the breeding grounds⁶. Habitat Breeding The species breeds on large clear freshwater lakes^{1, 2}, pools¹, the upper reaches of rivers^{1, 2} and streams² in the boreal, montane² and temperate forest zones⁴. It requires waters with a fairly high productivity of fish² surrounded by mature hard-wood trees with holes excavated by woodpeckers or natural cavities for nesting in². Non-breeding The species winters on large unfrozen lakes, rivers, lagoons, brackish waters and marshes^{1, 3}, generally avoiding highly saline waters³ although it may move to estuaries, coastal lagoons and sheltered sea coasts with waters less than 10 m deep in particularly harsh winters⁵. Diet Its diet consists predominantly of fish¹ less than 10 cm long², but may also include aquatic invertebrates (such as molluscs, crustaceans, worms, and adult and larval insects), amphibians, small mammals and birds¹. Breeding site The species nests in holes excavated by large woodpeckers or natural cavities in mature hardwood trees^{1, 2} with entry holes more than 15 m above the ground². It shows a preference for cavities with openings c.12 cm wide and internal diameters of c.25 cm⁴ in trees close to or up to 1 km away from water⁶. When natural tree-nesting sites are not available⁴ the species will use artificial nestboxes^{1, 2} or may nest among tree roots in undercut banks, on cliff ledges², in rock clefts⁷ or in dense scrub or loose boulders on islands². Sometimes several females may nest in the same tree³, especially on islands that provide suitable nesting sites in lacustrine or coastal locations⁴. Management information Artificial nestboxes commonly used are 85-100 cm high with openings 50-60 cm from the base⁴.

Threats The species is subject to persecution by anglers and fish-farmers who accuse it of depleting fish stocks¹, and it is occasionally drowned in freshwater fishing nets with mesh sizes greater than 5 cm (China)¹¹. The species is also threatened by the degradation of freshwater lakes through drainage and petroleum pollution in Russia⁸ and as a result of acid rain in North America². The species is susceptible to avian influenza so may be threatened by future outbreaks of the virus⁹. Utilisation The species is hunted in North America² and Russia⁸ (although it is not a popular game bird)², and its eggs used to be (and possibly still are) harvested in Iceland¹⁰.

References 1. del Hoyo et al. (1992). 2. Kear (2005b). 3. Madge and Burn (1988). 4. Johnsgard (1978). 5. Scott and Rose (1996). 6. Snow and Perrins (1998). 7. Flint et al. (1984). 8. Grishanov (2006). 9. Melville and Shortridge (2006). 10. Gudmundsson (1979). 11. Quan et al. (2002).

Further web sources of information

Detailed species account from Birds in Europe: population estimates, trends and conservation status (BirdLife International 2004)

Text account compilers Stuart Butchart (BirdLife International), Rob Calvert (BirdLife International), Jonathan Ekstrom (BirdLife International), Lucy Malpas (BirdLife International)

Contributors Stefan Pihl

IUCN Red List evaluators Jeremy Bird (BirdLife International), Stuart Butchart (BirdLife International)

Recommended citation BirdLife International (2010) Species factsheet: Mergus merganser. Downloaded from http://www.birdlife.org on 30/7/2010

This information is based upon, and updates, the information published in BirdLife International (2000) Threatened birds of the world. Barcelona and Cambridge, UK: Lynx Edicions and BirdLife International, BirdLife International (2004) Threatened birds of the world 2004 CD-ROM and BirdLife International (2008) Threatened birds of the world 2008 CD-ROM. These sources provide the information for species accounts for the birds on the IUCN Red List.

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