Mergus serratorred-breasted merganser

Geographic Range

Red-breasted mergansers have a holarctic distribution; they are found throughout much of the northern hemisphere. Red-breasted mergansers have distinct breeding and wintering ranges, although they overlap somewhat in northern, coastal areas. In the Americas they breed from Alaska throughout northern, boreal Canada to the maritime provinces and into the northern United States: Minnesota, Wisconsin, Michigan, and Maine. They breed in Greenland and Iceland and in Eurasia from the Faroe Islands, Ireland, and Scotland through Scandinavia, northern Russia and Asia to Siberia and the Kamchatka Peninsula. They may also breed in northeastern China, northern Japan, and as far south as northern Germany, Lake Baikal, Manchuria, and the Sea of Okhotsk. Red-breasted mergansers winter in coastal areas, including the Atlantic, Pacific, Gulf of Mexico, Great Lakes coasts, and other large, inland waterways as far south as northern Mexico in the Americas and the Baltic, North, Mediterranean, Black, Caspian, and Aral Seas in Eurasia. They sometimes wander as far south as portions of the Red Sea and to the Hawaiian Islands in winter. They are found throughout the year in northern coastal areas, including Iceland, parts of the British Isles, southeastern Alaska and the Aleutian Islands, coastal areas of Maine and the Canadian maritime provinces, and the northernmost lower peninsula of Michigan and northern shore of Lake Michigan. (Titman, 1999)


Red-breasted mergansers are found on wetlands and open bodies of freshwater, brackish, or saltwater in their breeding and wintering ranges. In the breeding range, they are found in the tundra and boreal zones. In winter and during migration they are found on protected waters along sea coasts and large, inland lakes and rivers, although they also use fast-flowing rivers. Red-breasted mergansers are found foraging mainly in shallow waters with submergent vegetation, although they also forage in deep waters, just as long as there is an abundance of their fish prey. (Titman, 1999)

Physical Description

Red-breasted mergansers are relatively large diving ducks with long, thin bills lined with serrated edges to help in capturing fish prey. Males are larger than females. Lengths range from 51 to 64 cm and weights from 800 to 1350 g. In their breeding plumage, males are more colorful, with dark greenish heads, a white collar, brown-speckled breasts, steel-gray flanks, and greenish-black backs that are bordered by a white patch. Both females and males have an asymmetrical crest of plumes at the back of their heads. Females are grayish brown overall, with a small, white wing bar, a whitish breast with gray speckles, and the head is cinnamon brown. There is an inconspicuous white eye ring. The bill and legs are reddish-orange and the bill has a black tip. Female plumage stays the same throughout the year and immature birds resemble females. Males in the non-breeding season resemble females but have wider, white wing bars. (Titman, 1999)

  • Sexual Dimorphism
  • male larger
  • male more colorful
  • Range mass
    800 to 1350 g
    28.19 to 47.58 oz
  • Range length
    51 to 64 cm
    20.08 to 25.20 in


Red-breasted mergansers are seasonally monogamous, but there is good evidence that extra-pair copulations may be frequent. Pairs may form as early as November, but most pair bonds form during spring migration, starting in March. Males use a courtship display and call to attract females. Usually several males display around a single female in an attempt to win her favor. Males hold their heads close to their body with the crest raised and their bill pointing up, they then do 1 of 2 alternate displays: the "head shake" and the "salute curtsy." The head shakes involves flicking the head from side to side. In the salute curtsy the male drops the bill forward, then rapidly flicks it up while straightening his neck and raising the chest above the water, the chest is then dropped back into the water, this may also be accompanied by kicking. A "yeow" call is used during the salute portion of the curtsy salute display. Females use a display that incites male courtship behavior, making a bobbing motion through the water as she holds her bill downwards. (Titman, 1999)

Red-breasted mergansers are relatively late breeders. Mated pairs arrive on the breeding grounds in May, egg-laying occurs in early June in the northernmost portions of the breeding range, with hatching in July and fledging in September to October. Females choose nests on land close to water, usually in dense vegetation or under objects, such as driftwood or boulders. Either an object or dense tree branches or grass forms a roof over the nest. Nests are usually within 23 m of the water, never more than 70 m. Females start the nest as a scrape, but gradually add grass and feathers as incubation progresses. They lay from 5 to 24 beige to gray eggs (mean 9.5), laying 1 egg every other day. They begin to incubate the eggs when the last egg is laid. Incubation is generally for 30 to 31 days, young hatch synchronously. Young fledge at 60 to 65 days after hatching. Because they breed relatively late, second clutches are unlikely. Most red-breasted mergansers mate first in their third year, although they are mature in their second year. (Titman, 1999)

  • Breeding interval
    Red-breasted mergansers breed once yearly.
  • Breeding season
    Red-breasted mergansers breed in May and June.
  • Range eggs per season
    5 to 24
  • Average eggs per season
  • Range time to hatching
    30 to 31 days
  • Range fledging age
    60 to 65 days
  • Range time to independence
    7 (high) weeks
  • Range age at sexual or reproductive maturity (female)
    2 to 3 years
  • Average age at sexual or reproductive maturity (female)
    3 years
  • Range age at sexual or reproductive maturity (male)
    2 to 3 years

Females incubate the eggs and brood and care for the young until they abandon them within a few weeks after hatching. Males abandon females on the nest soon after she begins incubating the eggs. (Titman, 1999)

  • Parental Investment
  • precocial
  • pre-fertilization
    • provisioning
    • protecting
      • female
  • pre-hatching/birth
    • provisioning
      • female
    • protecting
      • female
  • pre-weaning/fledging
    • protecting
      • female


The oldest recorded red-breasted merganser was 9 years and 4 months old. A female was also recorded breeding when she was 8 years old. Like many animals, most red-breasted merganser hatchlings do not survive through their first year. Up to 50% of hatchlings die because of exposure to cold weather, another 25% are preyed on. It is thought that about 50% of red-breasted mergansers survive migration and winter to breed the following year. (Titman, 1999)

  • Range lifespan
    Status: wild
    9.33 (high) years


Red-breasted mergansers are migratory, with distinct breeding and wintering ranges. They are active during the day and are highly aquatic. They have rapid, efficient flight and can swim and dive well by propelling themselves with their feet. Dives can be up to 44 seconds long and they can dive to depths of 9.2 meters. They cannot walk well because their feet are so far back on the body. Red-breasted mergansers spend about 50% of their waking hours in foraging activities, although this varies with the availability of prey. They are highly social and are typically seen in groups, with the exception of breeding season, when pairs separate to mate and nest. They migrate in small groups of 5 to 15, but in their fall migration they may gather in large groups of up to 15,000. They do not defend territories, even during the breeding season. Red-breasted mergansers also commonly associate with other birds, both in feeding and nesting areas. (Titman, 1999)

Home Range

Red-breasted mergansers are gregarious and do not defend territories. Females return to nest in the area where they were hatched. (Titman, 1999)

Communication and Perception

Red-breasted mergansers use visual displays and vocalizations in their courtship rituals. They also produce alarm calls that sound like "garr" or "grack." Males produce a drumming sound with their wings during copulation. (Titman, 1999)

Food Habits

Red-breasted mergansers eat mainly small fishes (10 to 15 cm long) and crustaceans. Their diet is usually made up of over 75% small fish, with less than 25% made up of crustaceans and other aquatic animals, such as insects, worms, and amphibians. They seem to prefer foraging in shallow water, but they will hunt wherever prey is abundant. Red-breasted mergansers forage in several different ways. They float at the surface, looking underwater as they go, they dive in deep or shallow water to search for prey, or they dive in formation with other red-breasted mergansers to herd schooling prey. This cooperative foraging strategy can be very effective and has been observed when mergansers are hunting sheepshead minnows. Other preferred fish prey include killifishes, sticklebacks, Atlantic salmon, sculpins, herring and their eggs, salmon eggs, silversides, and blueback herring. (Titman, 1999)

  • Animal Foods
  • amphibians
  • fish
  • eggs
  • aquatic or marine worms
  • aquatic crustaceans


A wide variety of predators feed on eggs and nestlings of red-breasted mergansers, including common ravens, great black-backed gulls, herring gulls, parasitic jaegers, and mink. Adults have been taken by great horned owls and gyrfalcons. They may also be taken by red foxes and snowy owls. (Titman, 1999)

  • Anti-predator Adaptations
  • cryptic

Ecosystem Roles

Red-breasted mergansers are important predators of small fish in their wetland habitats. Several bird species take advantage of the fact that red-breasted mergansers will herd fish prey to the water's surface when they are foraging. Snowy egrets, Bonaparte's, and ring-billed gulls will wait at the surface to grab fish scared by merganser foraging. Red-breasted mergansers are also attracted to areas where gulls are feeding on schooling fish. (Titman, 1999)

Red-breasted mergansers are parasitized by at least 60 kinds of parasitic worms, including Eustrongylides species, which may cause die-offs. They are also parasitized by ectoparasites, such as lice (Anaticola crassicornis, Anatoecus dentatus, Anatoecus icterodes, Holomenopon loomisi, Pseudomenopon species, and Trinoton querquedulae). (Titman, 1999)

Mutualist Species
Commensal/Parasitic Species

Economic Importance for Humans: Positive

Red-breasted mergansers are occasionally hunted, but they are not a common game bird. (Titman, 1999)

  • Positive Impacts
  • food

Economic Importance for Humans: Negative

Red-breasted mergansers are sometimes attracted to fish hatcheries and other commercial fish raising programs, as well as important salmon spawning streams. They are sometimes persecuted because of their predation on salmon parr (young salmon). (Titman, 1999)

  • Negative Impacts
  • crop pest

Conservation Status

Red-breasted mergansers have a wide distribution and large populations, they are not considered currently threatened. However, some populations may be threatened by wetland destruction and contamination by pesticides and lead. They are also captured in fishing nets fairly frequently. (Titman, 1999)

Other Comments

Mergus species, along with Lophodytes cucullatus (hooded mergansers) are most closely related to goldeneyes (Bucephala) and smews (Mergellus albellus). (Titman, 1999)


Tanya Dewey (author), Animal Diversity Web.


Arctic Ocean

the body of water between Europe, Asia, and North America which occurs mostly north of the Arctic circle.

Atlantic Ocean

the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.

World Map


living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

Pacific Ocean

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.

World Map


living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

World Map


uses sound to communicate

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

brackish water

areas with salty water, usually in coastal marshes and estuaries.


an animal that mainly eats meat


uses smells or other chemicals to communicate


the nearshore aquatic habitats near a coast, or shoreline.


having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.

  1. active during the day, 2. lasting for one day.

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.


an area where a freshwater river meets the ocean and tidal influences result in fluctuations in salinity.


A substance that provides both nutrients and energy to a living thing.


forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.


mainly lives in water that is not salty.


a distribution that more or less circles the Arctic, so occurring in both the Nearctic and Palearctic biogeographic regions.

World Map

Found in northern North America and northern Europe or Asia.


offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).


makes seasonal movements between breeding and wintering grounds


Having one mate at a time.


having the capacity to move from one place to another.


specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.


reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.


an animal that mainly eats fish


the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

seasonal breeding

breeding is confined to a particular season


reproduction that includes combining the genetic contribution of two individuals, a male and a female


associates with others of its species; forms social groups.


uses touch to communicate


Coniferous or boreal forest, located in a band across northern North America, Europe, and Asia. This terrestrial biome also occurs at high elevations. Long, cold winters and short, wet summers. Few species of trees are present; these are primarily conifers that grow in dense stands with little undergrowth. Some deciduous trees also may be present.


that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).


A terrestrial biome with low, shrubby or mat-like vegetation found at extremely high latitudes or elevations, near the limit of plant growth. Soils usually subject to permafrost. Plant diversity is typically low and the growing season is short.


uses sight to communicate

young precocial

young are relatively well-developed when born


Titman, R. 1999. Red-breasted merganser (Mergus serrator). The Birds of North America Online, 443: 1-20. Accessed April 02, 2009 at