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.
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