Common Merganser (Mergus merganser)

The common merganser is a large duck of rivers and lakes native to Europe, Asia, and North America. A seasonal migrant to the Chicago area often observed in the Chicago River system, its habitat is wooded lakes, rivers, and bays mainly around fresh water at all seasons.

There are three subspecies, differing in only minor detail:

• M. m. americanus – found in North America. Bill broader-based than in M. m. merganser, and a black bar crossing the white inner wing (visible in flight) on males.
• M. m. merganser – found throughout northern Europe and northern Asiatic Russia.
• M. m. orientalis – found in the Central Asian mountains. Slightly larger than M. m. merganser, with a slenderer bill.

These ducks have serrated edges to their bills to help them grip their prey, so they are often known as "sawbills". In addition to fish, they take a wide range of other aquatic prey, such as mollusks, crustaceans, worms, insect larvae, and amphibians.  

These are large, long-bodied ducks with thin, pointed wings. Their bills are straight and narrow, unlike the wide, flat bill of a “typical” duck. Males and females are sexually dimorphic, meaning that the two sexes vary in appearance.

Adult males are crisply patterned with gleaming white bodies and dark, iridescent-green heads for most of the year. The back is black and the bill red.

Females and immature birds are gray-bodied with a white chest and rusty-cinnamon heads with a shaggy crest. From late summer to mid-autumn, males wear a nonbreeding plumage that looks very similar to female plumage. In flight, both sexes show large white patches on the upper wings.

The common merganser forages by diving and swimming underwater, propelled by its feet, stroking with both feet in unison. The bird finds most food by sight and may swim along surface, dipping its head underwater repeatedly until prey is spotted, then diving in pursuit.

Courtship displays of male include swimming very rapidly in circles near female, suddenly stretching neck upward, pointing bill straight up, and giving soft call. The nest site is near water, usually in large tree cavity but also in crevices in rock, holes under tree roots or undercut banks. 

After the chicks leave the nest in summer, the female stays with them as they grow up while males gather in flocks. In winter, mergansers form large flocks on inland reservoirs and rivers. They stay in these tight flocks to feed and court during the cold months. During migration and winter, they mix with other fish-eating, diving ducks such as Bufflehead, goldeneyes, and other species of mergansers.

Migration: Common mergansers migrate mostly in small groups. Adult males, on average, seem to winter farther north than females and young. Migration is late in fall and early in spring.

Conservation status: stable

Sources: The Cornell Lab of Ornithology and National Audubon Society

Photo courtesy The Cornell Lab All About Birds