sea duck vs eider duck

sea duck and eider duck both are nouns.

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

As nouns, eider duck is a hyponym of sea duck; that is, eider duck is a word with a more specific, narrower meaning than sea duck:

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

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