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.
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