- Measure 22–27 inches (56–69 cm)
- Male has flashing white sides, green head, white breast, and long, thin red bill
- Female has gray body and sides; reddish-brown crested head sharply set off from white throat
- Red-breasted mergansers are similar, but males have gray sides, white neck ring, and rust-coloured breast; female has reddish-brown head that blends into gray of neck
Breeds on wooded rivers and ponds; winters mainly on lakes and rivers, occasionally on salt water
9–12 pale buff or ivory eggs in a down-lined tree cavity or sometimes on the ground or in an abandoned hawk’s nest.
Breeds across Canada from eastern Alaska, Manitoba, and Newfoundland south in mountains to California, northern New Mexico, Great Lakes, and northern New England. Winters south to northern Mexico, Gulf Coast states, and Georgia (rarely farther). Also in Eurasia.
Low rasping croaks.
Although preferring to feed on lakes, common mergansers are often driven to rivers by cold weather; there they are found in flocks of 10 to 20 birds, all facing upstream and diving in pursuit of fish. The narrow bill, with a hooked upper mandible and fine, saw-like teeth along the edges, is specialized to catch slippery fish. Pairs are formed in late winter, and until then one is likely to find flocks composed entirely of males or females.