eider duck vs sea duck

eider duck and sea duck both are nouns.

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

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

eider duck: duck of the Northern Hemisphere much valued for the fine soft down of the females
sea duck: any of various large diving ducks found along the seacoast: eider, scoter, merganser

eider duck (noun)	sea duck (noun)
duck of the Northern Hemisphere much valued for the fine soft down of the females	any of various large diving ducks found along the seacoast: eider, scoter, merganser