merganser vs sea duck

merganser and sea duck both are nouns.

Word	Noun	Adjective	Verb	Adverb
merganser	Yes	No	No	No
sea duck	Yes	No	No	No

As nouns, sea duck is a hypernym of merganser; that is, sea duck is a word with a broader meaning than merganser:

merganser: large crested fish-eating diving duck having a slender hooked bill with serrated edges
sea duck: any of various large diving ducks found along the seacoast: eider, scoter, merganser

merganser (noun)	sea duck (noun)
large crested fish-eating diving duck having a slender hooked bill with serrated edges	any of various large diving ducks found along the seacoast: eider, scoter, merganser