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< >  Mergus serrator - Red-breasted merganser (Click photographs/illustrations: full picture & further details)
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General and References

Alternative Names (Synonyms)

Fish duck
Mittelsäger (German)
Harle huppé (French)
Serrata mediana (Spanish)
Mergánsar de pecho rojo (Spanish)
Middleste Zaagbek (Dutch)
Småskrake (Swedish)

Names for newly-hatched

Duckling, downy.

Names for non-breeding males or other colour-phases


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Species Author

Debra Bourne

Major References

B1, B2, B3, B6, B8, B19, B25, B26, B27.

Aviculture information:
J23.13.w4, J23.13.w8
B7, B29, B31, B40, B94, B96, B97, B128.w1, B129
D1, D8

(UK Contacts)

(Further Reading)
Click image for full contents list of ELECTRONIC LIBRARY

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TAXA Group (where information has been collated for an entire group on a modular basis)

Parent Group

Specific Needs Group referenced in Management Techniques

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Aviculture Information


General information:
  • Seaducks are generally winter-hardy and sociable. They are preferably kept on a large area of clean, cold, deep water, at least some of which (preferably half the area) should be more than 60cm and preferably more than1m deep. As with other diving ducks, most species are relatively ungainly on land and ponds should have shallow sloping banks. Some cover along the pond edges will generally be appreciated. Preferred nesting sites vary greatly within this group, from open ground nesting to thick vegetation and tree holes.
  • Diets of grain, pellets fish and seafood may be used, also bread. These ducks generally need a higher-protein diet than most waterfowl species and high-protein pelleted diets specifically designed for seaducks are now available, although supplementation with fish may still be important particularly for breeding.
  • Feeding in troughs containing stones may avoid the development of overgrown bills. Provision of salt water may decrease the incidence of fungal and other infections.
  • Ducklings may be given high-protein starter crumbs and live food, and provided with access to deep water for swimming from an early age.
  • Mergansers, also known as "sawbills" and "fish ducks", require a large area of deep, clean water, with marginal vegetation providing cover along the edges. Very active and playful, the larger species may be belligerent and upset other ducks in a mixed collection. Breeders pellets, floating pellets and addition fish such as sprats or sand eels should be fed. These species are at particular risk from Foreign Body Ingestion, due to their tendency to pick up, play with and occasionally swallow, a wide variety of objects. Sawbills will nest in boxes or barrels, on the ground or raised, sometimes at a distance from water. Ducklings should be provided with access to swimming water from an early age.

(J23.13.w4, J23.13.w8, B7, B29, B40, B94, B129, D1)

Species-specific information:

  • Red-breasted mergansers should be provided with a large area of clear water, at least 80cm deep, with access to ice-free water in winter; an area with natural food available is advantageous, particularly for breeding. They may be kept with other species, at least on larger areas. Pellets, wheat and duckweed should be provided as well as fish.
  • Not common in collections and not easy to breed, these mergansers are bred sporadically. Fairly low raised nest boxes should be provided; ground-level nest boxes and dense natural ground cover near water are also used for nesting. Nests some distance from water may be used. Nest box dimensions of 12x12x14inches (30x30x35cm) with a 4 inch (10cm) entrance hole is suggested (B128.w1). Eggs are usually laid April to May. Ducklings should be reared with access to water for swimming and diving (B31).
  • Hybridisation has been reported with Lophodytes cucullatus - Hooded merganser, Tadorna tadorna - Common shelduck, Anas platyrhynchos - Mallard and others.

(J23.13.w8, B29, B30, B31, B40, B96, B97, B128.w1, B129).

Aviornis UK Ringing Scheme recommended average ring size: M 12.0mm (D8).

Individual Techniques linked in Wildpro

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External Appearance (Morphology)

Measurement & Weight

Length 19-26 inches 48-00cm (B3); 52-58cm (B1)
Adult weight General 780-1350g (B1).
Male Average 1200g maximum 1314g (B3); mean 2.6 lbs. (B8).
Female Average 925g maximum 1268g (B3); mean 2.1 lbs. (B8)
Newly-hatched weight --
Growth rate --

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Adult Bill Male Red with black culmen (dorsal stripe base to nail) and nail.
Variations (If present) Female: duller red with dusky culmen and nail
Eyes (Iris) Male Red.
Variations(If present) Reddish-brown.
Juvenile Bill Brownish-red.
Eyes (Iris) Pale brown.

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Adult Male Red.
Variations (If present) Female: duller red.
Juvenile Yellow-brown.

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Adult Male Head and upper neck glossy green-black with ragged crest, lower neck white with black dorsal stripe. Upper breast cinnamon with black spotting, sides of breast black with series white patches down sides from black mantle. Flanks, tail, tailcoverts, rump, ventral region vermiculated grey, blending to centre of abdomen white. Upperparts black centrally, white laterally: inner scapulars black, outer scapulars white.

Wings black outer feathers and leading edge, white secondaries and their coverts with black bars across bases of greater coverts and bases of secondaries.

Variations (If present) Female: Upper head dark brown from line through eyes, lower head and neck paler cinnamon shading to chin, throat and foreneck white running into central underparts white. Sides of breast and flanks mottled grey. Tail and rump grey, upperparts darker mottled grey. Wings dark grey with white, black-based secondaries and greater coverts.

Eclipse: Similar to female but darker mantle and male wing pattern retained.

Juvenile Similar to female but darker grey and with a shorter crest.

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Newly-hatched Characteristics

General: Upperparts brownish, underparts white.
Bill: Dark horn.
Feet: Olive brown.

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Reproductive Season

Time of year Begins April-June.
No. of Clutches One brood, but re-nests if clutch lost.

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Nest placement and structure

Well concealed on ground or in burrow or natural cavity, grass lined with down.

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Egg clutches

No. of Eggs Average 8-10 (B1, B8)
Range 6-14 (B1); 4-14 (B8).
Egg Description Buff or green-buff.

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31-32 days (B1); 29-35 days mean 31-32 days (B8).

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60-65 days (B1, B8).

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Sexual Maturity

Males Two years old.
Females Two years old.

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Feeding Behaviour

Adults Scan with head submerged, then dive; also feed on surface.
Newly-hatched --

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Parental Behaviour

Nest-building Solitary nests or in small groups, some local colonies. Built by female only.
Incubation By female.
Newly-hatched Tended by female, brooded initially at night and may be carried on the back.

Brood amalgamation occurs, often with only one female left tending. Independent before fledging.

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Social Behaviour

Intra-specific Gregarious throughout the year. Mainly found in moderate-sized flocks.
Inter-specific --

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Sexual Behaviour

Seasonal pair bond, male usually leaves during incubation.

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Predation in Wild


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Activity Patterns

Roost communally, often at sea.
Circadian Diurnally active.

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Natural Diet


Small shoaling fish, aquatic invertebrates, some plant material.

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Range and Habitat

Distribution and Movement (Migration etc.)


Northern North America south to Great Lakes; Greenland, Iceland, Northern Eurasia as far south as Britain, north-eastern China, northern Japan.

London: In the London Area, "scarce winter visitor and passage migrant." In 2000, about 29 individuals seen, at sites such as Dartford Marsh, and various reservoirs, also in Inner London one bird on The Serpentine in Hyde Park one day in December. (J322.65.w1)

Partly migratory. Remain across north-western Europe, Iceland and western Greenland all year. Also Atlantic and Pacific coasts North America, Mediterranean basin, southern part former USSR, eastern China, Korea Japan.

Males remain closer to breeding areas than do females.

Occasional and Accidental

Occasionally further south.

Accidentals reported Spitsbergen, Jan Mayen, Israel, Jordan, Iraq, Cyprus, Malta, Morocco, Azores, Madeira.



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Deep lakes and small rivers, preferably in wooded country. Winters at sea (inshore and offshore) including estuaries, coastal lagoons.

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Intraspecific variation


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Conservation Status

Wild Population -

Abundant (B1).

CITES listing --
Red-data book listing --
Threats Persecuted by sport fishermen and fish farmers (B8).

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Captive Populations

Rare in American collections (B8).

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