Common mergansers are the largest mergansers and the largest North American inland ducks. Males are slightly larger, on average. Males and females are colored quite differently. Males have bright, vivid, solid areas of color with sharply defined edges; black back, dark green to black head (changes to brown in non-breeding season), serrated red bill, red feet, and white body with slight peach-colored tinge on breast, and fading to some grey in the tail. Male common mergansers have no visible crest. Males can be readily distinguished from male common goldeneyes, which have shorter necks, rounded bodies, more ruffled heads, and white eyes.
Female common mergansers have the same red feet and bill as males, but they have a larger crest, a brown head, and the body is a less sharply defined mixture of grey and white, fading to white in the breast. The line between the brown neck and white breast is sharply defined, which distinguishes female common mergansers from similar female red-breasted mergansers.
Common mergansers or goosanders occur in both the Nearctic and Palearctic regions. Although their abundance has been decreasing in North America, they are still the most abundant of the mergansers found there. In North America, common mergansers may breed as far north as southern Alaska and Canada. Some occur year-round in the northern and western United States. North American birds winter in New England, midwestern and southern states, and the Pacific coast of the U.S. and Canada.
Common mergansers in the Palearctic region typically breed in northern Europe, Scandinavia, throughout Russia, and in much of northern Asia. Populations in Europe winter along the coasts of Scandinavia and northern Europe, as well as parts of the northern Mediterranean coast and north Africa. In Asia common mergansers winter in southern Asia, northern India, Japan, China, and Korea.
Common mergansers prefer to live in wooded areas along streams and rivers or near small, inland lakes. They can also be found along the shores of the Great Lakes, as well as on coastal streams in British Columbia. Nests are typically in a crevice of a deciduous tree along the shore, but sometimes will be in other types of crevices or on the ground, under tangled bushes. Mergansers may also occupy abandoned hawk nests when available.
Common mergansers are diving predators who locate their prey by sight, and therefore tend to feed in clear waters, less than 4 m deep, including estuaries, coastal bays, lakes, streams, and rivers. In the winter, common mergansers have been known to dive deeper in order to capture schooling fish. (Brewer, et al., 1991; Ehrlich, et al., 1988; Terres, 1980; Udvardy, 1977)
Common mergansers form mated pairs that last for at least one mating season and may last over a number of mating seasons. Pairs are formed in late winter, when a male circles a female and displays with his neck stretched forward and head feathers erect.
Common mergansers breed once per year, in the summer. They lay from 9 to 12 eggs between May and June. Eggs are about 64 mm long and have a pale yellow, ivory, or white buff appearance. After hatching, young birds follow their mothers to feeding sites. Young can swim and feed easily as soon as they leave the nest, although for the first few days most food is taken from the surface of the water. Within about 8 days, the young are skilled divers. Mothers abandon their young before they learn to fly, usually 30 to 50 days after hatching. Young mergansers join other young after being abandoned; mixed broods of more than 40 young have been observed.
Males do not help with raising the young. Females incubate the eggs for 28 to 35 days. The young are able to get around on their own soon after hatching, they leave the nest within 24 to 48 hours. When the nest is in a tree cavity, they leave by jumping to the ground. For the next few weeks, females can be seen leading their chicks to feeding sites, once they arrive there, the young are responsible for finding their own food. The mother may give warnings of potential dangers, but otherwise the young are mostly independent at feeding sites. They are skilled divers within 8 days of leaving the nest. Mothers abandon their offspring before the young birds develop the ability to fly.
The record for the oldest common merganser is held at 13 years 5 months, but banding records indicate maximum longevities of approximately 12 years 6 months for males and 13 years 10 months for females. (Mallory and Metz, 1998)
Common mergansers are social, preferring to be in groups of up to 75 individuals. Nest sites are usually isolated, but females sometimes nest close to others. Broods often intermix, producing mixed broods of up to 40 or more young, tended by one or more adult females. In the winter, huge gatherings of hundreds of individuals have been observed.
Common mergansers fly individually, in pairs, or in small flocks. They typically fly at about 70 km/h, and hold the head, body, and legs in a straight line. Females easily maneuver between trees in search of nesting sites, but they usually fly low over water or high when moving between bodies of water or during migration.
Common mergansers swim along the surface of clear shallow waters, paddling with their feet and with their heads submerged to look for prey. They make a small leap before diving into the water. When swimming under water they use their feet for propulsion. Common mergansers can remain underwater for up to 2 minutes, but maximum recorded dive times are 52 seconds in 2 to 3 m water and 37 seconds in 18 to 37 m water, with a typical dive lasting less than 30 seconds.
Common mergansers usually nest by themselves, but they have also been known to nest together, with up to 10 females in a single tree. Males sometimes drive other mergansers away from their favorite places, but they are otherwise not territorial. Common mergansers travel over fairly large areas to feed in medium to large bodies of clear water. Females will lead young up to 8 km downstream in rivers to feed in larger bodies of water.
Common mergansers use their excellent eyesight to find prey underwater while swimming on the surface. They can also find prey by probing underwater crevices when the water is cloudy. Males and females are mostly silent, but males make some calls during courtship, including hoarse croaking sounds and a twang somewhat like a guitar string. The females have a harsh karr, karr call that is sometimes heard during courtship or to warn others of a predator. Sometimes 2 or more birds dive from flight into the water with a particular body posture, producing an audible hollow sound, have been reported, but the purpose is unknown.
Common mergansers are skilled diving predators, eating mainly slower small fish. Their serrated bills are well suited for capturing small fish and other aquatic prey. Clear water is preferred for feeding because the birds hunt primarily by sight. When fish are scarce (usually in spring), mergansers will substitute other small aquatic prey such as insects, frogs, and snails. They have been known to eat aquatic plants sometimes, and to feed on trout from freshwater streams when nearby ponds and lakes freeze. These birds feed most actively in the early morning after dawn, in the afternoon, and before sunset.
Common mergansers are top predators in aquatic food chains. Predation on adults is not common and young mergansers have good survival rates. This may be mostly because they breed far enough north that most of the common nest predators are not a threat to the eggs and hatchlings. The nest predators that live far enough north to potentially be a threat to Mergus merganser include red squirrels, American martens, northern flickers, and black bears. Large predatory fish such as northern pike, along with predatory birds such as bald and golden eagles, common loons, and some owls and hawks, may prey on more vulnerable, immature mergansers in the water. However, no predators of common mergansers have been documented.
Common mergansers are considered keystone predators, acting to control the populations of many inland fish such as perch and bass. By thinning these populations, it is thought that mergansers help to keep their populations healthy and growing. Wilderness lakes where mergansers feed generally produce larger, healthier fish than lakes without predation.
Brood parasitism, or egg-dumping, is fairly common between individual common mergansers, as well as between common mergansers and other related species. Common mergansers have been known to place their eggs in the nests of common goldeneyes and hooded mergansers. And both common goldeneyes and hooded mergansers have also been known to place their eggs in the nests of common mergansers. This can lead to mixed clutches of up to 19 eggs.
Common mergansers have no known negative effects on humans.
By thinning fish populations in lakes and ponds, common mergansers allow the remaining fish to thrive, because they have less competition for the habitat's limited resources. Thus, more fish reach larger sizes for anglers. Common mergansers are also attractive birds that attract ecotourism.
Common mergansers remain common in both the Palearctic and Nearctic regions, and are not thought to be threatened at this time. Common merganser populations are most limited by the availability of nesting sites, such as tree cavities. Nest boxes installed by humans have improved populations in some areas.
Tanya Dewey (editor), Animal Diversity Web.
Russell Becker (author), Kalamazoo College, Ann Fraser (editor, instructor), Kalamazoo College.
Brewer, R., G. McPeek, R. Adams. 1991. The Atlas of Breeding Birds of Michigan. East Lansing, MI: Michigan State University Press.
Ehrlich, P., D. Dobkin, D. Wheye. 1988. The Birder's Handbook. New York, NY: Simon & Schuster Inc.
Mallory, M., K. Metz. 1998. Common Merganser. Pp. 1-28 in A Poole, F Gill, eds. The Birds of North America, Vol. 12:442, First Edition. Philadelphia, PA: The Birds of North America, Inc.
Pough, R. 1951. Audubon Water Bird Guide. Garden City, NY: Doubleday & Company, Inc.
Terres, J. 1980. Merganser, common. Pp. 195, 210-211 in The Audubon Society Encyclopedia of North American Birds, Vol. 1, First Edition. New York: Alfred A. Knopf, Inc.
Udvardy, M. 1977. The Audubon Society Field Guide to North American Birds. New York: Alfred A. Knopf, Inc.