Red-breasted mergansers are 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 a double crest of plumes at the back of their heads. Females are grayish brown, with a small, white wing bar, a whitish breast with gray speckles, and the feathers on the head are reddish brown. The bill and legs are reddish-orange and the bill has a black tip. Females 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.
Red-breasted mergansers are found throughout much of the northern hemisphere. They have separate breeding and wintering ranges, although they are found year-round in some northern, coastal areas. In the Americas they breed from Alaska throughout northern Canada to the Atlantic ocean 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. 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.
Red-breasted mergansers are found on wetlands and open bodies of freshwater, brackish, or saltwater. In summer they are found in northern areas, usually coasts, lakes, and rivers near conifer forests or tundra. In winter and during migration they are found on protected waters along sea coasts and large, inland lakes and rivers. Red-breasted mergansers forage mainly in shallow waters with submergent vegetation and abundant fish prey.
Red-breasted mergansers form mated pairs each season. 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.
Red-breasted mergansers breed relatively late in the season. Mated pairs arrive on the breeding grounds in May and lay eggs in early June. Young hatch in July and can fly by September or October. Females choose nests on land close to water, usually in dense vegetation or under objects. Nests are usually within 23 m of the water. Females start the nest as a scrape, but gradually add grass and feathers throughout incubation. They lay from 5 to 24 beige to gray eggs. 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.
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)
The oldest recorded red-breasted merganser was 9 years and 4 months old. Like many animals, most 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.
Red-breasted mergansers migrate between summer breeding areas and areas where they spend the winter. They are active during the day. They have rapid, efficient flight and can swim and dive well by propelling themselves with their feet. They cannot walk well because their feet are so far back on the body. Red-breasted mergansers spend about 50% of their waking hours foraging. They are highly social and are usually seen in groups, except during breeding season, when pairs separate to mate and nest. They migrate in spring 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 are commonly seen foraging and nesting near other bird species.
Red-breasted mergansers are gregarious and do not defend territories. Females return to nest in the area where they were hatched. (Titman, 1999)
Red-breasted mergansers use visual displays and calls during the breeding season to attract mates. They also produce alarm calls that sound like "garr" or "grack."
Red-breasted mergansers eat mainly small fishes (10 to 15 cm long) and crustaceans. Their diet is usually made up of more than 75% small fish, with less than 25% made up of crustaceans and other aquatic animals, including 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. Preferred fish prey include killifishes, sticklebacks, Atlantic salmon, sculpins, herring and their eggs, salmon eggs, silversides, and blueback herring.
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)
Red-breasted mergansers are important predators of small fish in their wetland habitats. 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.
Red-breasted mergansers are sometimes attracted to fish hatcheries and important salmon spawning streams. They are sometimes persecuted because of their predation on young salmon.
Red-breasted mergansers are occasionally hunted, but they are not a common game bird. (Titman, 1999)
Red-breasted mergansers have a wide distribution and large populations, they are not considered currently threatened. Some populations may be threatened by wetland destruction and contamination by pesticides and lead. They are also captured in fishing nets sometimes.
Tanya Dewey (author), Animal Diversity Web.
Titman, R. 1999. Red-breasted merganser (Mergus serrator). The Birds of North America Online, 443: 1-20. Accessed April 02, 2009 at http://bna.birds.cornell.edu/bna/species/443.