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Common Merganser
Mergus merganser
– Family
Authors: Mallory, Mark, and Karen Metz
Revisors: Pearce, John

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Adult male Common Merganser, Edmonton, AB, 22 April.
Figure 1. Distribution of the Common Merganser in North America.

This large, cold-hardy, fish-eating sea duck nests worldwide near large lakes and rivers in northern forested habitats. In North America, it winters on large lakes, rivers, and reservoirs of the Pacific Northwest, Rocky Mountains, central United States, Great Lakes region and along the coasts as far north as Alaska. Also known as Goosander, a name sometimes restricted to Eurasian populations, the species has a variety of popular North American names that refer to mergansers in general, names such as sawbill, fish duck, and sheldrake. In flight, this duck has a flat, pointed profile. It generally nests in tree cavities (or nest boxes) but will nest on the ground or in crevices. Females breed in their second year, generally lay one clutch per year, and will lay their white eggs in the nests of other cavity-nesting ducks.

As a top predator in aquatic food chains, this species has served as an indicator of environmental health both for contaminants (pesticides, toxic metals) and lake acidification. Because this merganser was thought to threaten salmon and trout stocks, its diet has been intensively studied, and in some regions it has been the target of eradication programs. Although we lack a reliable estimate of its current population size, trend data suggest that the North American population is generally stable or increasing, with many northeastern US populations expanding south into former range.

The species has been studied extensively in breeding areas of Atlantic Canada (White 1957, Erskine 1971a, 1972), British Columbia (Munro and Clemens 1937, Wood 1985a, 1985b), and in wintering habitats in the central United States (Salyer and Lagler 1940, Huntington and Roberts 1959, Anderson and Timken 1971, McCaw et al. 1996). Diet of this species is well understood, as are its foraging and courtship behavior, growth, plumage molts, and relationships to pollution. Despite its broad distribution and importance as a bio-indicator of aquatic food-chain degradation, however, we know little about the nesting and reproductive biology of this duck, or its population size and dynamics.