The diet of the common merganser consists mainly of small fish, but it will also take insects, molluscs, crustaceans, worms, amphibians, and even small mammals and birds (4) (5) (7) (9). Adult common mergansers have sometimes been recorded swallowing fish up to 30 centimetres or more in length (5) (7). Most feeding takes place during the day (5) (8).
The common merganser swims low in the water (5) (6), often repeatedly dipping its head beneath the surface until prey is spotted. It then dives and pursues its prey underwater (4) (5) (7) (8). The common merganser’s legs are positioned far back on its body, making it an efficient swimmer but awkward at walking on land (2) (5).
A gregarious species, the common merganser often feeds in groups of up to 75 individuals outside of the breeding season. Large flocks also come together to roost, sometimes numbering several thousand birds (5) (8) (9).
In some areas, the breeding season of the common merganser begins around late March, but it may occur somewhat later further north (8) (9). Courtship displays often involve several pairs (5), and include the male swimming rapidly in circles near the female before stretching the neck and bill upwards and calling (7). The common merganser is generally monogamous (5).
Somewhat unusually for a duck, the common merganser nests in cavities in trees, either in natural hollows or in holes made by woodpeckers. If suitable tree holes are not available, this species will also use artificial nest boxes, cliff ledges, rock crevices, hollow logs, holes among tree roots, or even old buildings (4) (5) (7) (9). The nest cavity may be lined with down from the female’s breast (4) (5) (7). The common merganser occasionally breeds in loose colonies, with several females nesting together in the same tree (5) (8) (9).
The common merganser lays between 6 and 17 creamy-white eggs (4) (5), which are incubated by the female for 28 to 35 days (5) (7). Female common mergansers will often lay eggs in the nests of other females, or in the nests of other cavity-nesting ducks. Other ducks, such as the hooded merganser (Lophodytes cucullatus) and common goldeneye (Bucephala clangula), also sometimes lay their eggs in the nests of this species (5).
Young common mergansers are well developed at hatching, and leave the nest hole at just one or two days old (4) (5) (7). The young jump to the ground from the nest, and are led to water by the female (5). Although the female common merganser cares for the chicks for several weeks, the chicks catch their own food, initially eating aquatic insects before starting to eat fish from about 12 days old (4) (5) (7). The female abandons the young before they become capable of flight at about 60 to 75 days old (5).
Male common mergansers leave the females early on in the incubation period and gather in flocks to moult, often travelling considerable distances to suitable waters (8) (9). This species often remains around its breeding grounds for as long as open, unfrozen water allows (8), with the main migration to the wintering areas usually occurring between October and December (9).
The female common merganser has been recorded breeding from two years old, typically laying one clutch a year. This species has been known to live for up to 13 years (5).