Editing Common merganser (section)

==Behaviour==

===Feeding===
As with other Mergansers, these piscivorous 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 [[mollusc]]s, [[crustacean]]s, worms, insect larvae, and amphibians; more rarely, small mammals and birds may be taken.<ref name=Hoyo/><ref name=Madge/> As in other birds with this trait, the salmon-pink colouration shown variably by males is probably dietary-derived from the [[carotenoid]] pigments found in some crustaceans and fish.<ref name=Hudon1990/> When not diving for food, they are usually observed swimming on the surface of the water or resting on rocks in the middle of rivers or hidden in the vegetation along riverbanks, or (in winter) on the edge of floating ice.<ref name=Hoyo/><ref name=Madge/>

===Habits===
In most places, the common merganser is equally at home in salt and fresh water. In larger streams and rivers, they float down with the current for a few miles and either fly back or, more commonly, fish their way back, diving all the 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 a deep pool at the foot of a waterfall or rapid. When swimming leisurely, they position themselves in the water much like dabbling ducks, but they also swim deep in the water like cormorants, especially when swimming upstream. They often sit on a rock in the middle of the water, similar to cormorants, often with their wings half-open to the sun. To rise from the water, they flap along the surface for many yards. Once they are airborne, their flight is strong and fast.<ref name=Hume1880/> 

They often fish in a group forming a semicircle and driving the fish into shallow water, where they are easy to catch. Their normal vocalisation is a low, harsh croak, but during the breeding season, males and juveniles produce a plaintive, soft whistle. They are generally wary, and one or more birds will remain on sentry duty to warn the flock of approaching danger. If disturbed, they will often disgorge food before moving.<ref name=fbi6/> Although they move clumsily on land, they resort to running when pushed to do so, assuming a very upright position similar to penguins, and frequently falling and stumbling.<ref name=Baker1922/>

===Breeding===
[[Bird nest|Nesting]] is usually in a tree cavity, so the species requires mature forest as its breeding habitat; they also readily use large nest boxes where these are available, requiring an entrance hole {{convert|15|cm|in|frac=2|abbr=on}} in diameter.<ref name=duFeu2005/> In treeless areas such as the mountains of Central Asia, they use holes in cliffs and steep, high banks, sometimes a considerable distances from the water.<ref name=fbi6/> The female lays 6–17 (most often 8–12) white to yellowish eggs, and raises one brood per season. The ducklings are carried by the mother on her back to rivers or lakes immediately after hatching, where they feed on freshwater invertebrates and small fish fry, fledging at 60–70 days of age. The young are sexually mature at two years of age.<ref name=Hoyo/><ref name=Madge/><ref name=Snow/> Common mergansers are known to form [[Crèche (zoology)|crèches]], with single females having been observed with over 70 ducklings at one time. <ref name=Mervosh2018/>

===Movements===
The species is a partial [[bird migration|migrant]], with birds moving away from areas where rivers and large lakes freeze in the winter, but resident where waters remain open. Birds from Eastern North America migrate south in small groups to the United States wherever lakes and rivers are ice-free; on the milder Pacific coast, they are permanent residents. [[Scandinavia]]n and [[Russia]]n birds also migrate southwards, but western European birds, and a few in [[Japan]], are largely resident.<ref name=Hoyo/><ref name=Madge/> In some populations, the males also show distinct moult migration, leaving the breeding grounds as soon as the young hatch to spend the summer (June to September) elsewhere. In particular, 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.<ref name=Snow/><ref name=Little1985/><ref name=Hatton2004/>