Common merganser (Mergus merganser)
|Also known as:||American goosander, American merganser, Asiatic goosander, Asiatic merganser, eastern goosander, Eurasian goosander, Eurasian merganser, fish duck, goosander, sawbill, sheldrake|
|Size||Length: 58 - 72 cm (2) (3)|
Wingspan: 82 - 100 cm (3)
|Weight||900 - 2,160 g (4)|
- The common merganser’s long, narrow beak has tooth-like serrations that help it grasp slippery prey.
- The common merganser has been recorded swallowing large fish up to 30 cm or more in length.
- Unusually for a duck, the common merganser nests in holes in trees.
- Female common mergansers will often lay their eggs in the nests of other females, or even of other species.
- The common merganser’s legs are set far back on its body, making it an efficient swimmer but awkward at walking on land.
The common merganser is classified as Least Concern (LC) on the IUCN Red List (1).
Also known as the goosander, the common merganser (Mergus merganser) is a large, slender diving duck with a streamlined body (4) (5) (6). It is the largest of the mergansers or ‘sawbills’ (2) (3), a group of fish-eating ducks with long, narrow, serrated beaks that are ideally adapted for grasping slippery prey (4) (5) (7). The beak of the common merganser is bright red-orange with a black tip, and has a slight hook at the end (3) (4) (5) (8).
The male and female common merganser are quite different in appearance. The breeding male has a large, rounded, glossy green-black head, which contrasts with the white lower neck, breast, belly and flanks. The underparts may also have a pale salmon-pink tinge. The lower back and tail are grey, the upper back and outer half of the upperwing are black, and there is a large white patch on the inner part of the wing (2) (3) (4) (5) (6) (8). The male common merganser has bright red feet and legs (3) (5) (8) and a slight bushy ‘mane’ of feathers down the back of the neck (3) (8).
The female common merganser is slightly smaller than the male (3) (8) and differs in having a reddish-brown head and upper neck, a long, shaggy crest, a slightly duller beak, and largely greyish upperparts and wings (2) (3) (4) (5) (8). A white patch on the wing is visible in flight (3) (4) (5). The underparts of the female common merganser are grey, with a creamy-white belly and a distinct white patch on the chin (3) (4) (5) (8).
Outside of the breeding season, the male common merganser more closely resembles the female, but is darker on the back, whiter on the flanks, and retains a larger white patch on the wing (2) (3) (4) (5) (8). The non-breeding male also has a short, ragged crest on the head, but this is shorter than in the breeding female (2) (4) (5). The breeding and non-breeding plumage of the female common merganser are quite similar (2) (5), but outside of the breeding season the crest becomes shorter and there is usually a pale line between the beak and eye (3) (8).
Juvenile common mergansers resemble the adult female, but are duller overall and have a whitish throat, paler head and yellowish bill (2) (3) (5) (8). Full adult plumage is not attained until the second year of life (3) (8). Ducklings have a brown to reddish-brown head, white cheeks and underparts, and dark brownish upperparts with white patches on the rump and wings (2) (5).
Three subspecies of common merganser are usually recognised: the Eurasian merganser or goosander (Mergus merganser merganser); the common or American merganser (Mergus merganser americanus); and the Asiatic goosander (Mergus merganser orientalis). M. m. americanus is distinguished by a deeper, less hooked beak, and also has a narrow black bar across the white wing patch of the male. M. m. orientalis is slightly larger than M. m. merganser and has a shorter, more slender beak (2) (3) (5) (8).
Although usually relatively quiet, the male common merganser produces a twanging sound, a harsh croak and a bell-like note during courtship, while the female may make a harsh ‘gruk’ or ‘karr’ (2) (3) (4) (5) (8). The alarm call of this species is a harsh, croaking ‘grrk’, and the female may also hiss if cornered in the nest or with young (2) (5).
The common merganser is widespread across the whole of the northern hemisphere, occurring in Europe, Asia and North America. Its breeding range includes northern Europe and Scandinavia, central Asia, Russia, western China, Japan, and Canada and the northern United States (9).
Many northern populations move south in winter, into the United States, Europe, central Asia, the east coast of China, Korea and Japan. However, in some areas the common merganser is resident year-round or migrates only short distances (5) (9).
As their common names suggest, the Eurasian merganser (M. m. merganser) occurs in Europe and Asia, the Asiatic goosander (M. m. orientalis) occurs in Asia, and the American merganser (M. m. americanus) occurs in Canada and the United States (2) (3) (8).
A largely freshwater species, the common merganser breeds along large, clear rivers and lakes bordered by forest (4) (7) (8) (9). In addition to waters containing plenty of fish, it also requires surrounding forests with mature trees that are suitable for nesting (5) (9).
In winter, the common merganser typically uses large, unfrozen lakes, rivers, lagoons and reservoirs (4) (5) (6) (7) (9). It may also inhabit marshes, coastal bays, estuaries and brackish waters (4) (5) (7) (9), but generally tends to avoid salt water (8) (9).
The diet of the common merganser consists mainly of small fish, but it will also take insects, molluscs, crustaceans, worms, amphibians, and even small mammals and birds (4) (5) (7) (9). Adult common mergansers have sometimes been recorded swallowing fish up to 30 centimetres or more in length (5) (7). Most feeding takes place during the day (5) (8).
The common merganser swims low in the water (5) (6), often repeatedly dipping its head beneath the surface until prey is spotted. It then dives and pursues its prey underwater (4) (5) (7) (8). The common merganser’s legs are positioned far back on its body, making it an efficient swimmer but awkward at walking on land (2) (5).
A gregarious species, the common merganser often feeds in groups of up to 75 individuals outside of the breeding season. Large flocks also come together to roost, sometimes numbering several thousand birds (5) (8) (9).
In some areas, the breeding season of the common merganser begins around late March, but it may occur somewhat later further north (8) (9). Courtship displays often involve several pairs (5), and include the male swimming rapidly in circles near the female before stretching the neck and bill upwards and calling (7). The common merganser is generally monogamous (5).
Somewhat unusually for a duck, the common merganser nests in cavities in trees, either in natural hollows or in holes made by woodpeckers. If suitable tree holes are not available, this species will also use artificial nest boxes, cliff ledges, rock crevices, hollow logs, holes among tree roots, or even old buildings (4) (5) (7) (9). The nest cavity may be lined with down from the female’s breast (4) (5) (7). The common merganser occasionally breeds in loose colonies, with several females nesting together in the same tree (5) (8) (9).
The common merganser lays between 6 and 17 creamy-white eggs (4) (5), which are incubated by the female for 28 to 35 days (5) (7). Female common mergansers will often lay eggs in the nests of other females, or in the nests of other cavity-nesting ducks. Other ducks, such as the hooded merganser (Lophodytes cucullatus) and common goldeneye (Bucephala clangula), also sometimes lay their eggs in the nests of this species (5).
Young common mergansers are well developed at hatching, and leave the nest hole at just one or two days old (4) (5) (7). The young jump to the ground from the nest, and are led to water by the female (5). Although the female common merganser cares for the chicks for several weeks, the chicks catch their own food, initially eating aquatic insects before starting to eat fish from about 12 days old (4) (5) (7). The female abandons the young before they become capable of flight at about 60 to 75 days old (5).
Male common mergansers leave the females early on in the incubation period and gather in flocks to moult, often travelling considerable distances to suitable waters (8) (9). This species often remains around its breeding grounds for as long as open, unfrozen water allows (8), with the main migration to the wintering areas usually occurring between October and December (9).
The female common merganser has been recorded breeding from two years old, typically laying one clutch a year. This species has been known to live for up to 13 years (5).
The common merganser has an extensive range, and its populations are thought to be stable or increasing (5) (9). However, it may face a number of threats. In some areas, the common merganser is persecuted by anglers and fish-farmers in the belief that it competes for fish stocks (3) (5) (9), and it is sometimes accidentally drowned in fishing nets (9). It is not a popular game bird, but is still often shot in parts of North America and Russia (5) (9). The common merganser’s eggs have sometimes been harvested in Iceland (9).
Degradation of freshwater lakes through drainage, pollution and acid rain threatens the common merganser’s breeding grounds (5) (9), while its migration and wintering grounds are being affected by river channelisation, the loss of wetlands, and increased sedimentation from agriculture and industry (5). Forestry practices and logging may also reduce the availability of suitable nest cavities, further limiting this species’ populations (5).
A potential future threat to the common merganser comes from outbreaks of avian influenza, to which this species is susceptible (9).
As a top predator in its habitat, the common merganser is considered to be an important indicator of the health of aquatic ecosystems, and has been involved indirectly in management plans to reduce pollution (5). In North America, programmes to install artificial nest boxes have not directly targeted the common merganser, but it is likely to benefit from boxes placed for other species (5).
Recommendations for future studies that would aid in the conservation and management of the common merganser include investigating its breeding requirements, winter habitats and population size (5).
Find out more about the common merganser and its conservation:
BirdLife International - Common merganser:
The Cornell Lab of Ornithology: All About Birds - Common merganser:
Birds of North America Online - Common merganser:
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- Avian influenza: also known as “bird flu”, a contagious disease caused by any strain of influenza virus that is carried by and primarily affects birds.
- Brackish: slightly salty water, usually a mixture of salt and freshwater, such as that found in estuaries.
- Crustaceans: diverse group of animals with jointed limbs and a hard chitinous exoskeleton, characterised by the possession of two pairs of antennae, one pair of mandibles (mouthparts used for handling and processing food) and two pairs of maxillae (appendages used in eating, which are located behind the mandibles). Includes crabs, lobsters, shrimps, woodlice and barnacles.
- Incubate: to keep eggs warm so that development is possible.
- Molluscs: a diverse group of invertebrates, mainly marine, that have one or all of the following; a horny, toothed ribbon in the mouth (the radula), a shell covering the upper surface of the body, and a mantle or mantle cavity with a type of gill. Includes snails, slugs, shellfish, octopuses and squid.
- Monogamous: having only one mate during a breeding season, or throughout the breeding life of a pair.
- Moult: periodic shedding of (usually) the outermost body covering (such as feathers, fur or skin) during growth and development, or at specific times of the year.
- Subspecies: a population usually restricted to a geographical area that differs from other populations of the same species, but not to the extent of being classified as a separate species.
IUCN Red List (May, 2012)
- Kear, J. (2005) Ducks, Geese and Swans. Volume 2: Species Accounts (Cairina to Mergus). Oxford University Press, Oxford.
- Ogilvie, M.A. and Young, S. (2002) Photographic Handbook: Wildfowl of the World. New Holland Publishers, London.
The Cornell Lab of Ornithology: All About Birds - Common merganser (May, 2012)
Mallory, M. and Metz, K. (1999) Common merganser (Mergus merganser). In: Poole, A. (Ed.) The Birds of North America Online. Cornell Lab of Ornithology, Ithaca. Available at:
- Peterson, R.T., Mountfort, G. and Hollom, P.A.D. (1993) Collins Field Guide: Birds of Britain and Europe. HarperCollins Publishers, London.
- Kaufman, K. (2001) Lives of North American Birds. Houghton Mifflin Harcourt, Boston, Massachusetts.
- Madge, S. and Burn, H. (1988) Wildfowl. Christopher Helm Publishers, London.
BirdLife International - Common merganser (May, 2012)