Common Merganser is a fish-eating duck that rides low in the
water on freshwater lakes and rivers. The male in breeding plumage
has a white belly, breast, and flanks with a gray rump and black
back. The crestless head is dark green, the bill is red, and
the eye is dark. The female Common Merganser has a solid gray
body with a reddish-brown head. She has a narrow, red, serrate
bill and white chin-patch at the base of the lower mandible.
The short crest of the female is often not obvious. The male
in non-breeding plumage is similar to the female, with some white
on his back. Juveniles are also similar, but have a white bar
across the face.
Common Mergansers prefer fresh water in all seasons. They use
deep, clear, forested lakes, reservoirs, and rivers for breeding.
In winter, they occupy similar habitat, as well as bays, coastal
estuaries, and harbors. They frequent salt-water habitats in
Washington in the winter.
Mergansers often form large rafts, or floating flocks, in the
winter. They dive and swim under water, finding most of their
food by sight.
make up the majority of the diet of adult birds. Aquatic insects
are a primary source of food for young birds. Mussels, crustaceans,
and other aquatic creatures are also eaten.
usually start breeding in their second year. Pair bonds form
in late winter or early spring. Nests are located near water,
usually in a large tree cavity, but may also be in rock crevices,
under tree roots, and in nest boxes. Nests are made of wood chips
and other debris found in the cavity. Down is added by the female.
The female usually lays 9 to 12 eggs, although nest parasitism
is common. Incubation lasts from 28 to 35 days, and is done by
the female alone. The male usually remains near the nest until
incubation begins, and rarely stays with the brood once they
hatch. The young jump from the cavity within a day or so of hatching
and head for the water. The female leads the young to food, and
they feed themselves. The young feed mostly by dabbling at first,
but after about a week can dive well. Females usually abandon
the young before they can fly, sometimes as early as the end
of the first week. Broods often join together in crèches
after they are about a week old.
Common Mergansers migrate short to medium distances in small
groups. The post-breeding molt migration of males is not well
understood, but it is thought that they migrate north to large
lakes, bays, and rivers. Females molt alone after they leave
their broods. The migration of the Common Merganser takes place
late in the fall and early in the spring. Adult males seem to
winter farther north than females and juvenile birds.
appear to be stable across North America, and the Christmas Bird
Count has reflected a slight increase in Washington in recent
years. Common Mergansers are an important indicator of the health
of wetlands. Habitat degradation can remove nesting cavities.
River channelization increases sediment in the water. Agriculture
and industry often alter water quality. All of these affect the
Common Merganser, as well as many other species. Common Mergansers
have come under fire for preying on salmon, but studies in British
Columbia show that the most common prey item there for Common
Mergansers is the sculpin.