In addition to fish, they take a wide range of other aquatic prey, such as molluscs, crustaceans, worms, insect larvae, and amphibians; more rarely, small mammals and birds may be taken.

[8] When not diving for food, they are usually seen swimming on the water surface, or resting on rocks in midstream or hidden among riverbank vegetation, or (in winter) on the edge of floating ice.

The ducklings are taken by their mother on her back to rivers or lakes immediately after hatching, where they feed on freshwater invertebrates and small fish fry, fledging when 60–70 days old.

[13] The species is a partial migrant, with birds moving away from areas where rivers and major lakes freeze in the winter, but resident where waters remain open.

Eastern North American birds move south in small groups to the United States wherever ice-free conditions exist on lakes and rivers; on the milder Pacific coast, they are permanent residents.

[5][6] In some populations, the males also show distinct moult migration, leaving the breeding areas as soon as the young hatch to spend the summer (June to September) elsewhere.

Notably, most of the western European male population migrates north to estuaries in Finnmark in northern Norway (principally Tanafjord) to moult, leaving the females to care for the ducklings.

[16] In February 2020, a rare common merganser sighting was documented in Central Park, New York; the bird was in obvious distress, with its beak being trapped by a piece of debris.

[17] Within western Europe, a marked southward spread has occurred from Scandinavia in the breeding range since about 1850, colonising Scotland in 1871, England in 1941, and also a strong increase in the population in the Alps.