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Which Anseriformes subfamily includes sea ducks such as eiders and mergansers?

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mallard — Anseriform Mallard (*Anas platyrhynchos*).

Sea ducks, including eiders and mergansers, belong to the subfamily Merginae within the family Anatidae of the order Anseriformes.1

While some classification systems categorize these birds as tribes (such as Mergini and Somateriini) within the subfamily Anatinae, other taxonomic frameworks recognize them as the distinct subfamily Merginae. This group consists of approximately 18 species, including scoters, goldeneyes, and the bufflehead, which are primarily saltwater divers.21