Mergus merganser
goosander;common merganser
(Also: common merganser)

Mergus merganser (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. ()

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, cylindrical 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.

Adults have an average basal metabolic rate of 638 kJ/d. Field metabolic rates, studied in autumn in Scotland, were found to be higher in females than males, with an overall average of 2.32 ml CO2/h. ()

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.

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. ()

Mergus merganser is a social species, preferring to be in groups of up to 75 individuals. Nest sites are usually isolated, but females sometimes nest in close proximity or even in the same tree. 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. Birds in flocks tend to maintain regular spacing in flight, but on the water large groups will distribute irregularly, with pairs remaining close. ()

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 one horizontal line. Females easily maneuver between trees in search of nesting sites, but flight is typically either low and along waterways or high and directional. ()

Common mergansers swim along the surface of clear shallow waters, paddling alternately, with their heads submerged in order to see their prey. Their dive is preceded by a slight leap. When diving, Mergus merganser individuals use only their feet for propulsion, paddling in unison. In larger groups, one individual may lead others by initiating a group dive, but in small groups individuals tend to dive independently. Common mergansers are capable of remaining 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 the typical dive lasting less than 30 seconds. ()

Home Range

Nesting sites are usually separated from one another, but common mergansers have also been known to nest in close proximity in some cases, with up to 10 females nesting in a single tree. Territorial behavior is minimal, with males occasionally defending favorite loafing sites near the nest. Mergus merganser individuals feed over a large range, seeking medium to large bodies of clear water. Females will lead young up to 8 km downstream in order to feed in larger bodies of water. ()

Common mergansers use their keen eyesight to locate prey underwater while swimming on the surface, but can also revert to probing underwater crevices when the water is cloudy. Males are typically only vocal during courtship, and have several calls, including hoarse croaking sounds and a twang somewhat like a guitar string. Females are also predominantly silent, vocalizing only in courtship or as a warning of danger. The females have a harsh karr, karr call. Aerial chases, in which 2 or more birds dive from flight into the water with a particular body posture to produce an audible hollow sound, have been reported, but their 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 in some instances, 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 generally considered to be top predators in aquatic food chains. Predation rates are very low, and their offspring survival rate is good, largely because Mergus merganser typically nests far enough north that most common nest predators are not a threat. 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. However, no predators of common mergansers have been documented. ()

Mergus merganser is considered a keystone predator, acting to control the populations of many inland fish such as perch and bass. By thinning these populations, it is thought that mergansers keep lakes and ponds below their carrying capacity. This allows those fish that survive to thrive and grow, whereas in lakes and ponds lacking the merganser predation, fish are often too prolific and bring about their own growth stagnation. Thus, wilderness lakes where mergansers feed generally produce larger, healthier fish than lakes without predation, where no individual fish grow very well because of overcrowding.

Brood parasitism, or egg-dumping, is fairly common between individual common mergansers, as well as between Mergus merganser and other related species. Common mergansers have been known to parasitize and be parasitized by both common goldeneyes and hooded mergansers. This has been shown to lead to mixed clutches of up to 19 eggs. ()

Common mergansers have no known negative effects on humans.

Fish that have to compete in overpopulated lakes and ponds often do not grow very large. 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 legal size for fishermen, who benefit indirectly from this form of population control provided by the mergansers. Common mergansers are also attractive birds that attract ecotourism attention. ()

Common mergansers remain common in both the Palearctic and Nearctic regions, and are not thought to be threatened at this time.

The limiting factor on Mergus merganser populations seems to be nesting site availability. Thus, urbanization and human civilization are thought to be the causes for common mergansers moving breeding sites farther south in the southeastern United States (recorded as far south as Virginia in the last 50 years) as former habitats are destroyed due to human development. However, nest boxes installed by humans have provided additional suitable nesting sites in some areas.

Although North American populations seem to have declined somewhat over the last 200 years, they remain large and may now be increasing. In Europe, deforestation is thought to be a cause of some breeding range reduction, but populations continue to thrive and grow throughout Eurasia, especially in Scotland, Ireland, and Wales. ()