Mergansers are streamlined birds that glide down streams or shallow beaches with ease. Common mergansers are big, long-bodied ducks with thin, pointed wings. Their bills aren't as wide or flat as those of "normal" ducks. The crests on the backs of females' heads are shaggy.
Males are beautifully patterned with glossy white bodies and dark, glittering-green heads throughout the year. The back is black, as is the bill. Females and young birds are gray-bodied with a white chest and rusty-cinnamon heads.
Mergansers are diving ducks that eat fish underwater. The female stays with the chicks after they fledge in summer, while males congregate in flocks. Mergansers form huge flocks on inland lakes and rivers throughout the winter. They remain in these close populations to feed and date throughout the cold season. They mix with other fish-eating, diving ducks such as Bufflehead, goldeneyes, and other species of mergansers during migration and the winter.
Mergansers lay 8-11 eggs, but 6-13 is not uncommon. Buff with a pale yellow tinge. Females sometimes deposit their eggs in each other's nests. Incubation is only carried out by the female, taking 30-35 days. After hatching, young may stay in the nest for a day or more; then they climb to the cavity's entrance and jump to the ground. Female adults take care of baby birds for several weeks, but youngsters feed themselves; they might even live if abandoned soon after. Young birds are able to fly about 65-70 days after hatching.
Mergansers are often confused with other types of ducks, but there are some key differences. For example, male common mergansers have a green head, while males of all other merganser species have a black head. Additionally, all other merganser species lack the shaggy crest that is found on female common mergansers. Finally, mergansers have a thin and serrated bill, while other types of ducks have a broader and flatter bill.