|Harlequin duck, Histrionicus histrionicus (male)|
As the name implies, most but not all, are essentially marine outside the breeding season. Many species have developed specialized salt glands to allow them to tolerate salt water, but these are absent from juveniles. Some of the mergansers prefer riverine habitats.
All but two of the 20 species in this group occupy habitats in far northern latitudes.
The fish-eating members of this group, such as the mergansers and Smew, have serrated edges to their bills to help them grip their prey. These are therefore often known as "sawbills".
Other seaducks take molluscs or crustaceans from the sea floor.
There are twenty living species in ten extant genera.
- Genus †Chendytes, the diving-geese. These birds became extinct in prehistoric times. They were large, goose-like ducks with reduced wings which were unfit for flying, but could assist in diving as in the great auk. At least one species survived to the Holocene.
- Genus Polysticta
- Steller's eider Polysticta stelleri
- Genus Somateria, the eiders. These are large marine ducks. The drakes have body plumage showing varying amounts of black and white, and distinctive head patterns. Females are brown.
- Genus Histrionicus
- Harlequin duck Histrionicus histrionicus
- Genus †Camptorhynchus
- Genus Melanitta, the scoters. These are stocky marine ducks. The drakes are mostly black and have swollen bills. Females are brown.
- Genus Clangula
- Genus Bucephala, the goldeneyes. These are less marine than some species in this group, and will winter on fresh water. Drakes have white bodies with black backs and distinctive head markings. Females are grey with chestnut heads.
- Genus Mergellus (sometimes included in Mergus)
- Smew Mergellus albellus
- Genus Lophodytes (sometimes included in Mergus)
- Hooded merganser Lophodytes cucullatus
- Genus Mergus, the typical mergansers. These are the least marine of this group, only red-breasted and common mergansers being common on the sea. These are large saw-billed ducks which dive for fish.
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