Common Merganser Life History

Habitat

Common Mergansers spend the breeding season in northern forested habitats near large lakes and rivers. Since they nest in cavities of large trees, breeding Common Mergansers are usually found in mature forests. They spend winters on large lakes, rivers, and reservoirs in the southern and coastal regions of their breeding range, and in additional wintering grounds across the northern and western United States. They tend to prefer freshwater wintering habitat over saltwater, but they may winter in coastal bays, estuaries, and harbors.Back to top

Food

Common Mergansers mostly eat fish, but they also eat aquatic invertebrates (including insects, mollusks, crustaceans, and worms), frogs, small mammals, birds, and plants. They forage in clear aquatic habitats such as streams, rivers, lakes, coastal bays, and estuaries. While they mainly hunt in waters less than about 13 feet deep, in the winter they sometimes venture into deeper waters where fish are schooling. They find their prey by sight, often probing sediments and underwater stones with their slender bills, which have sharp serrations for grasping slippery prey. Their diet is heaviest in fish during the winter, and includes salmon, trout, suckers, sculpin, shad, sunfish, sticklebacks, chub, minnows, and eels. During the breeding season they supplement their fish diet with caddisflies, mayflies, backswimmers, flies, water striders, dragonflies, crane flies, beetles, caterpillars, freshwater sponges, spiders, snails, mussels, and other invertebrates. Nestlings eat mostly aquatic invertebrates, switching to fish at about 12 days old.Back to top

Nesting

Nest Placement

The female chooses the nest site, which is usually in a natural cavity or woodpecker hole in a live or dead tree, up to 100 feet off the ground and within a mile of water. Common Mergansers nest less frequently in rock crevices, old sheds, chimneys, lighthouses, holes in banks, holes in the ground, hollow logs, and burrows. They readily nest in boxes, including those designed for the much smaller Common Goldeneye. Sometimes they nest on the ground.

Nest Description

The female shapes a nest bowl in the materials that have accumulated at the bottom of the nest cavity (such as wood chips and shavings or old squirrel nests), and may also add grasses. After laying eggs she lines the nest with downy feathers plucked from her own breast.

Nesting Facts

Behavior

Common Mergansers spend much of their time afloat, loafing, fishing, and often sleeping on open water. They may form flocks of up to 75 individuals. They often swim in small groups along the shoreline, dipping their heads underwater to search for prey and then diving with a slight leap. Often when one bird dives in a large group, the others follow the leader and disappear. They can stay under for up to 2 minutes, but they normally dive for less than 30 seconds. Males chase each other during communal courtship displays, sometimes bumping or striking each other. Females sometimes lay their eggs in other ducks’ nests, including other Common Mergansers as well as Hooded Mergansers or Common Goldeneyes. The male usually abandons the nest during incubation, and the female cares for the ducklings on her own. She escorts them from the small streams and ponds near the nest site to larger lakes, rivers, and bays downstream. After leaving the nest, the young are in danger from hawks, owls, Bald Eagles, Golden Eagles, Common Loons, and even fish such as northern pike, but they can escape from predators by running on the surface of the water or skulking under banks. Broods often join together in groups of multiple females with 40 or more young. Back to top

Conservation

Common Merganser populations in North America have held steady between 1966 and 2019, according to the North American Breeding Bird Survey. Partners in Flight estimates a global breeding population of 1.9 million and rates them 9 out of 20 on the Continental Concern Score, indicating a species of low conservation concern. Pesticides, toxic metals, and acid rain can degrade the habitat of Common Mergansers, thin their eggshells, and reduce their prey. Being at the top of the aquatic food chain, this species is particularly susceptible to such effects, and people sometimes use Common Mergansers as an indicator of environmental health. Though they are not prized as a game bird, some Common Mergansers are shot every year either for sport or by mistake. At times they have been targeted for eradication because they were thought to threaten salmon and trout stocks.

Credits

Lutmerding, J. A. and A. S. Love. (2020). Longevity records of North American birds. Version 2020. Patuxent Wildlife Research Center, Bird Banding Laboratory 2020.

Partners in Flight. (2020). Avian Conservation Assessment Database, version 2020.

Pearce, John, Mark L. Mallory and Karen Metz. (2015). Common Merganser (Mergus merganser), version 2.0. In The Birds of North America (P. G. Rodewald, editor). Cornell Lab of Ornithology, Ithaca, New York, USA.

Sauer, J. R., D. K. Niven, J. E. Hines, D. J. Ziolkowski Jr., K. L. Pardieck, J. E. Fallon, and W. A. Link (2019). The North American Breeding Bird Survey, Results and Analysis 1966–2019. Version 2.07.2019. USGS Patuxent Wildlife Research Center, Laurel, MD, USA.

Sibley, D. A. (2014). The Sibley Guide to Birds, second edition. Alfred A. Knopf, New York, NY, USA.

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