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The Red-Breasted Merganser is a medium-sized duck that can be found throughout the Northern Hemisphere. They may be identified by their red, serrated bills, the spiky crests on their heads and rust-colored breasts. Closely related to the other Merganser species, the Red-Breasted Merganser belongs to the subfamily of Merginae, or seaducks. Like many of its Merganser relatives, it summers in high, Northern latitudes such as Alaska, Northern Canada, Europe and Asia. During winter months it migrates South and tends to inhabit coastlines and saltwater estuaries. The Red-Breaster Merganser is a diving duck that can use its long, serrated bill to feed primarily on fish.
Like the other Mergansers, the Red-Breasted Merganser is a migratory duck that tends to breed in high, Northern latitudes. Unlike the other Mergansers, it tends to prefer marine environments when summering in lower latitudes. It can summer in riverine environments as well, such as lakes, rivers and streams, though this is less common. Another thing that tends to set the Red-Breasted Merganser apart from other ducks is that they can mate comparatively late in the season. Their young tend to not even fledge until early Fall, and have developed protections against the cold, Northern waters.
Adult Red-Breasted Mergansers are sexually dichromatic, meaning males and females have different coloring. Females have rust-colored heads and breasts, grey bodies and white undersides. Males can be colored similarly to females, though their plumage may change dramatically during mating season. The mating plumage of an adult male can include an iridescent green head, white neck and a chest that is darker red and speckled with black. Both genders have similar spiky crests and white spots under their wings that are visible only during flight. Juveniles tend to be similar in coloring to females.
The Red-Breasted Merganser is considered a diving duck and has a long, thin body and a serrated bill. Its body shape can help to facilitate diving, while the serrated bill can be used to grasp fish. This serrated bill has led to it being known as a "saw-bill" duck. This nickname is also shared with ducks such as the other Mergansers and the Smew. Bill serration is typically observed in duck species that feed primarily on fish. Ducks that feed on mollusks and other food sources will tend more toward the stereotypical smooth bill. Red-Breasted Mergansers, however, may also take prey such as insects, frogs and mollusks to supplement their diet.