Red-breasted Merganser (Mergus serrator)

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Image of a Red-breasted Merganser in its natural habitat
Birds Merganser

Introduction

The Red-breasted Merganser (Mergus serrator) is a medium-sized diving duck in the family Anatidae. It is a widespread Holarctic species, breeding across northern parts of North America, Europe and Asia and usually moving to coastal waters, estuaries and large inland lakes during winter. Its long, narrow, serrated bill is specialised for catching fish and other aquatic prey.

Physical Description

The Red-breasted Merganser measures approximately 51–64 cm in length, with a wingspan of about 70–86 cm. It has a streamlined body, a long neck, a narrow reddish bill with serrated edges, and a shaggy crest that is present in both sexes.

Breeding males have a dark green head, a conspicuous white collar, a rusty or reddish-brown, finely streaked breast, grey flanks and a dark back with white patches. Their eyes are red. Females are more subdued, with a cinnamon-brown head, grey-brown body, pale throat and reddish bill. Immature birds resemble females, while males in non-breeding plumage become less contrasting and may appear female-like, although they often retain broader white wing markings.

The species is slightly slimmer and more finely built than the Common Merganser. The bill is also narrower, and the head and neck pattern is generally less sharply defined.

Range and Habitat

The Red-breasted Merganser has a broad distribution across the temperate and subarctic Northern Hemisphere. It breeds in northern North America, including Alaska, Canada and parts of the northern United States, and across northern and temperate regions of Europe and Asia.

During the breeding season, it uses freshwater lakes, rivers, sheltered bays and other open waters in boreal forest, taiga and tundra regions. Nests are usually placed on the ground close to water, concealed by dense vegetation, driftwood, rocks or other cover.

Many populations migrate or disperse southwards after breeding. Wintering birds are commonly found along marine coasts, sheltered bays, estuaries and inshore waters, although some remain on large lakes, reservoirs and rivers where water remains open. The species is therefore associated with both freshwater and saltwater habitats, with coastal waters particularly important outside the breeding season.

Behaviour and Diet

Red-breasted Mergansers are active, agile swimmers and divers. They usually forage during the day and may occur alone, in pairs or in flocks. Large groups can gather during migration and winter, sometimes alongside other diving ducks and seabirds.

The species feeds mainly on small fish, which it pursues underwater. Crustaceans, aquatic insects, frogs, tadpoles and worms may also be taken. Ducklings feed more heavily on aquatic invertebrates. In shallow water, groups may sometimes cooperate by driving small fish towards the shore or into confined areas where they can be caught more easily.

Courtship includes swimming displays, head movements, neck stretching, wing movements and vocal or mechanical sounds. The female selects and prepares the nest, usually a shallow depression lined with plant material and down. A typical clutch contains about 7–10 eggs, although clutch size varies. The female alone incubates the eggs for roughly 29–35 days. The ducklings are precocial and leave the nest soon after hatching, following the female to water and feeding themselves.

Identification Tips

  • Look for a slim, long-bodied diving duck with a narrow, serrated bill and a shaggy rear crest.
  • In breeding plumage, the male has a dark green head, white collar, rusty streaked breast, grey flanks and red eyes.
  • The female has a cinnamon-brown head, a pale throat and a grey-brown body. The transition between the brown head and pale breast is usually less sharply defined than in a female Common Merganser.
  • The Red-breasted Merganser is commonly found on coastal bays, estuaries and open marine water in winter, whereas Common Mergansers are more often associated with freshwater habitats during that season.
  • Compared with the Common Merganser, the Red-breasted Merganser generally has a thinner bill, a less cleanly divided head-and-neck pattern and a more ragged-looking crest.

Conservation Status

The Red-breasted Merganser is listed globally as Least Concern on the IUCN Red List. Its very broad range and substantial population provide protection against immediate global extinction risk, although status and trends can differ between regions. In parts of Europe, for example, regional assessments have identified declines and increased conservation concern.

Potential pressures include degradation of coastal and freshwater wetlands, pollution, disturbance at breeding and wintering sites, reductions in fish prey, entanglement in fishing gear and the effects of climate change on aquatic ecosystems. Conservation measures include protecting important wetlands and coastal waters, maintaining clean and productive fish habitats, reducing pollution and disturbance, and monitoring populations at important migration and wintering sites.

Frequently Asked Questions

Where can the Red-breasted Merganser be found?

It occurs across much of the northern Hemisphere. It breeds in northern North America, Europe and Asia, then often moves to coastal waters, estuaries and large inland lakes during winter.

What does the Red-breasted Merganser eat?

It eats mainly small fish, supplemented by crustaceans, aquatic insects, frogs, tadpoles and worms. Ducklings consume proportionally more aquatic invertebrates than adults.

How can a Red-breasted Merganser be identified?

The key features are its slim shape, narrow serrated bill and shaggy crest. Breeding males have a green head, white collar and rusty streaked breast, while females are brown-headed with a pale throat and grey-brown body.

Is the Red-breasted Merganser a migratory bird?

Many populations are migratory or partly migratory. Birds breeding in northern regions commonly move south after breeding, while some populations remain year-round in milder coastal areas.

Where does the Red-breasted Merganser nest?

It usually nests on the ground near water, concealed by dense vegetation, driftwood, rocks or other cover. The female builds or lines a shallow depression and incubates the eggs.

Is the Red-breasted Merganser endangered?

Globally, it is classified as Least Concern by the IUCN. Local or regional populations may still face threats from wetland loss, pollution, disturbance, fishing-gear entanglement and changing food supplies.

References

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