Mergus

 ( Bird Genera ) 

The common merganser ( Mergus merganser) and red-breasted merganser ( M. serrator) have broad ranges in the northern hemisphere. The Brazilian merganser ( M. octosetaceus) is a South American duck, and one of the six most threatened waterfowl in the world, with possibly fewer than 250 birds in the wild. The scaly-sided merganser or "Chinese merganser" ( M. squamatus) is an endangered species. It lives in temperate East Asia, breeding in the north and wintering in the south.

The hooded merganser ( Lophodytes cucullatus, formerly known as Mergus cucullatus) is not of this genus but is closely related. The other "aberrant" merganser, the smew ( Mergellus albellus), is phylogenetics closer to goldeneyes ( Bucephala).

Although they are Mergini, most of the mergansers prefer riverine habitats, with only the red-breasted merganser being common at sea. These large piscivore typically have black-and-white, brown or green hues in their plumage, and most have somewhat shaggy crests. All have serrated edges to their long and thin bills that help them grip their prey. Along with the smew and hooded merganser, they are therefore often known as "sawbills". The goldeneyes, on the other hand, feed mainly on , and therefore have a more typical duck-bill.

Mergus ducks are also classified as "diving ducks" because they submerge completely in looking for food. In other traits, however, the genera Mergus, Lophodytes, Mergellus, and Bucephala are very similar: uniquely among all Anseriformes, they do not have notches at the hind margin of their sternum, but holes surrounded by bone.

	† Mergus australis	Auckland Island merganser	Auckland Islands, New Zealand (extinct c. 1902). The species identity of merganser bones from mainland New Zealand—North Island, South Island, and Stewart Island Islands—is unresolved. /ref>
	† Mergus milleneri	Mergus milleneri	Chatham Island, New Zealand. Extinct sometime after human settlement of the Chatham Islands, which began c. 1500.

• Mergus miscellus is known from the Middle Miocene Calvert Formation (Barstovian, c.14 million years ago) of Virginia, United States.
• Mergus connectens lived in the Early Pleistocene about 2–1 million years ago, in Central and Eastern Europe.

The Early Oligocene booby "Sula" ronzoni was at first mistakenly believed to be a typical merganser.Mlíkovský (2002b): p. 264 A Late Serravallian (13–12 million years ago) fossil sometimes attributed to Mergus, found in the Sajóvölgyi Formation of Mátraszőlős, Hungary, probably belongs to Mergellus. The affiliations of "Anas" albae from the Messinian (c. 7–5 million years ago) of Hungary are undetermined; it was initially believed to be a typical merganser too.Mlíkovský (2002b): p. 124

• Mlíkovský, Jirí (2002b): Cenozoic Birds of the World, Part 1: Europe. Ninox Press, Prague.