Mergus
Mergus is the genus of the typical mergansers, fish-eating ducks in the subfamily Anatinae. The genus name is a Latin word used by Pliny the Elder and other Roman authors to refer to an unspecified waterbird.
The common merganser and red-breasted merganser have broad ranges in the northern hemisphere. The Brazilian merganser 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" is an endangered species. It lives in temperate East Asia, breeding in the north and wintering in the south.
The hooded merganser, often termed Mergus cucullatus, is not of this genus but is closely related. The other "aberrant" merganser, the smew, is phylogenetically closer to goldeneyes.
Although they are seaducks, most of the mergansers prefer riverine habitats, with only the red-breasted merganser being common at sea. These large fish-eaters typically have black-and-white, brown and/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 mollusks, and therefore have a more typical duck-bill.
Mergus are also classified as "divers" because they go completely under water 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.
Extant species
| Image | Scientific name | Common name | Distribution |
| Mergus merganser | Common merganser or goosander | Europe, northern and central Asia, and North America | |
| Mergus octosetaceus | Brazilian merganser | Brazil | |
| Mergus serrator | Red-breasted merganser | Northern North America, Greenland, Europe, and Asia. | |
| Mergus squamatus | Scaly-sided merganser | East Asia |
Extinct species
- New Zealand merganser, Mergus australis
- Chatham Island merganser, Mergus milleneri, is known from Holocene remains from the Chatham Island.
- Mergus miscellus is known from the Middle Miocene Calvert Formation of Virginia, USA.
- Mergus connectens lived in the Early Pleistocene about 2–1 million years ago, in Central and Eastern Europe.