Common Merganser - Behaviour

Behaviour

Like the other mergansers, these fish-feeding ducks have serrated edges to their bills to help them grip their prey; they are therefore often known as "sawbills". In addition to fish, they take a wide range of other aquatic prey, such as molluscs, crustaceans, worms, insect larvae, and amphibians; more rarely, small mammals and birds may be taken. As in other birds with the character, the salmon-pink tinge shown variably by males is probably diet-related, obtained from the carotenoid pigments present in some crustaceans and fish. When not diving for food, they are usually seen swimming on the water surface, or resting on rocks in midstream or hidden among riverbank vegetation, or (in winter) on the edge of floating ice.

In most places, the Common Merganser is nearly as much a salt-water as a fresh-water frequenter. In larger streams and rivers, they float down with the stream for a couple of 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, or fishing vigorously in some deep pool near the foot of some waterfall or rapid. When floating leisurely, they position themselves in water similar to ducks. But they swim deep in water like Cormorants too, especially when swimming upstream. They often sit on some rock in the middle of the water, similar to Cormorants, often half-opening their wings to the sun. In order to rise from water, they flap along the surface for many yards. Once they are airborne the flight is strong and rapid. They often fish in as a group forming a semicircle and driving the fish into a shallow, where they are captured easily. Their ordinary voice is a low, harsh croak but during the breeding season they (including the young one) makes a plaintive, soft whistle. Generally, they are wary and one or more birds stay on sentry duty to warn the flock on the approach of danger. And when disturbed, they often disgorge food before moving. Though they move clumsily on land, they resort to running when pressed, assuming a very upright position similar to penguins, and falling and stumbling frequently.

Nesting is normally in a tree cavity, thus it requires mature forest as its breeding habitat; they also readily use large nest boxes where provided, requiring an entrance hole 15 cm 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. The ducklings are taken by their mother in her bill to rivers or lakes immediately after hatching, where they feed on freshwater invertebrates and small fish fry, fledging when 60–70 days old. The young are sexually mature at two years old.

The species is a partial migrant, with birds moving away from areas where rivers and major lakes freeze in the winter, but 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. In some populations, the males also show distinct moult migration, leaving the breeding areas as soon as the young hatch to spend the summer (June to September) elsewhere. Notably, most of the western European male population migrates north to estuaries in Finnmark in northern Norway (principally Tanafjord) to moult, leaving the females to care for the ducklings. Much smaller numbers of males also use estuaries in eastern Scotland as a moulting area.