The Red-breasted Merganser is a medium-sized “saw-bill” duck with a wide distribution in the Northern Hemisphere, breeding up to 75°N. An expert diver, it usually runs across the water to become airborne and can fly low and noiselessly over water at considerable speed. Its serrated mandibles enhance its ability to capture underwater prey. It is similar in distribution and ecology to the Common Merganser (Mergus merganser) but, in contrast, it occurs more frequently in salt water and estuaries, typically nests on the ground, and has dark-colored down.
Its spectacular asymmetrical nuchal crest appears incongruous with symmetry elsewhere in nature. Odd social courtship displays draw attention to males in late winter and spring. This duck does not acquire full breeding plumage until the second year, and it first breeds when at least 2 years of age. Seasonally it is a notably late breeder, having young that often do not fledge until September. Young are adapted to cool northern waters and quickly become independent. Females are left to incubate and care for young as males depart during early incubation for poorly known molting areas. Large creches are a feature of coastal areas with high densities of breeders.
The Red-breasted Merganser breeds and winters in cool northern environments with abundant small fish. It is a bird of the tundra and boreal forest zones, inhabiting coastal bays and large water bodies or rivers. In coastal areas, Red-breasted Mergansers are often observed at river mouths where, on islands, they breed in loose colonies and in association with terns and gulls. There are remarkable records of distribution of this species at high latitudes.
Much of what we know about the Red-breasted Merganser comes from studies conducted in Iceland and Scandinavia by researchers examining other species of diving ducks concurrently (Bengtson Bengtson 1970b , Bengtson 1971c , Bengtson 1971a , Bengtson 1971b , Bengtson 1972b ; Hildén 1964 , Nilsson Nilsson 1965 , Nilsson 1970a , Nilsson 1970b , Nilsson 1980 , Joensen 1973 , Sjöberg Sjöberg 1985 , Sjöberg 1988a , Sjöberg 1989 ). Early North American studies of food habits stemmed from attempts to measure impacts of mergansers on salmon stocks, and to protect fish (White White 1936 , White 1937 , White 1939a , White 1939c , White 1957 , Munro and Clemens 1939 , Elson Elson 1950 , Elson 1962 ).
More recently, efforts have been made to examine the level of competition between foraging Red-breasted Mergansers and sport fish in the Great Lakes region (Bur et al. 2008). Reproductive biology is known primarily from studies in eastern Canada, and remains largely unknown elsewhere. Work proceeding at Kouchibouguac National Park, New Brunswick, since 1984 is attempting to elucidate reproductive strategies that involve habitat selection, brood parasitism, and survival, in association with a large colony of terns (Young and Titman Young and Titman 1986 , Young and Titman 1988 , Titman 1997 , Craik and Titman 2008, 2009). Only recently have key post-breeding molting sites been identified and the biology of flightless birds been documented (Craik et al. 2009, 2011).
Red-breasted Mergansers exhibit lower levels of breeding population differentiation than the Common Merganser, possibly because of range expansion to more northerly latitudes following the last glacial maximum in North America. Additionally, costs of dispersal from breeding sites may be lower for the ground-nesting Red-breasted Merganser than for the Common Merganser, which requires a cavity for nest placement (Pearce et al. 2009).
Overall, the Red-breasted Merganser remains one of the least understood species of waterfowl in North America. Size and trends in continental populations are not reliably known because surveys do not typically differentiate between Red-breasted and Common mergansers, large portions of the species' range are not surveyed, and most counts occur too early to provide adequate estimates for this late-breeding duck. Aspects of migration and connectivity between breeding, molting, and wintering sites remain largely unknown.