7-8-2017 RIVER DANUBE, REGENSBURG - COMMON MERGANSER (Mergus merganser)

The Common merganser is a large seaduck that lives in forested areas of Europe, Asia, and North America. These birds eat mainly fish and nest in holes in trees.

The Common merganser has a crest of longer head feathers, but these usually lie smoothly rounded behind the head. Adult males in breeding plumage are easily distinguished; their body is white with a variable salmon-pink tinge, the head is black with an iridescent green gloss, the rump and tail are grey, and the wings are largely white on the inner half, and black on the outer half. Females and males in "eclipse" (non-breeding plumage) are largely grey, with a reddish-brown head, white chin, and white secondary feathers on the wing. Juveniles (both sexes) are similar to adult females but also show a short black-edged white stripe between the eye and bill. The bill and legs are red to brownish-red, brightest on adult males, and dullest on juveniles.

Common mergansers are found in Europe, the northern and central Palearctic, and North America. These birds are partial migrants; they leave areas where rivers and major lakes freeze in the winter but are resident where waters remain open. Eastern North American birds move south in small groups to the United States wherever ice-free conditions exist on lakes and rivers; on the milder Pacific coast, they are permanent residents. Scandinavian and Russian birds also migrate southwards, but Western European birds, and a few in Japan, are largely resident. Common mergansers breed in forested areas near freshwater lakes, rivers, pools, and streams. During the summer they inhabit lakes, rivers, reservoirs, brackish waters, and marshes. They try to avoid saltwater but may be found in estuaries, coastal bays, and lagoons.

Common mergansers are carnivores (piscivores). They feed mainly on fish but also hunt a wide range of other aquatic prey, such as mollusks, crustaceans, worms, insect larvae, and amphibians. More rarely, they will take small mammals and birds.

Common mergansers are diurnal birds that usually do their hunting in the early morning, in the afternoon, and before sunset. When not diving for food, they are usually seen swimming on the water surface, or resting on rocks in midstream; they may also hide among riverbank vegetation, or (in winter) on the edge of floating ice. In larger streams and rivers, Common mergansers float down with the stream for a few miles, and either fly back again or more commonly fish their way back, diving incessantly the whole way. In smaller streams, they are present in pairs or smaller groups, and they float down, twisting round and round in the rapids; they also fish vigorously in a deep pool near the foot of a waterfall or rapid. When floating leisurely, they position themselves in water similar to ducks, but they also swim deep in water like cormorants, especially when swimming upstream. They often sit on a rock in the middle of the water, similar to cormorants, often half-opening their wings to the sun. To rise from the water, they flap along the surface for many yards. Once they are airborne, the flight is strong and rapid. Their ordinary voice is a low, harsh croak, but during the breeding season, mergansers (including the young) make a plaintive, soft whistle. Generally, they are wary, and one or more birds stay on sentry duty to warn the flock of approaching danger. When disturbed, Common mergansers often disgorge food before moving. Though mergansers move clumsily on land, they resort to running when pressed; they assume a very upright position similar to penguins and frequently fall and stumble.

Common mergansers are monogamous and form pairs that usually last for at least one breeding season. They breed in the summer and nest normally in a tree cavity; they also readily use large nest boxes where provided, requiring an entrance hole 15 cm (5.9 in) in diameter. In places devoid of trees (like Central Asian mountains), they use holes in cliffs and steep, high banks, sometimes at considerable distances from the water. The female lays 6-17 (most often 8-12) white to yellowish eggs and raises one brood in a season. Incubation lasts 28 to 35 days done only by the female. The ducklings are born precocial and are taken by their mother in her bill to rivers or lakes immediately after hatching; there they feed on freshwater invertebrates and small fish fry. Fledging usually occurs when ducklings are between 60 and 70 days old and reproductive maturity is reached at the age of two years.