eider duck vs eider

eider duck and eider both are nouns.

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

As nouns, eider duck and eider are synonyms defined as:

eider duck and eider: duck of the Northern Hemisphere much valued for the fine soft down of the females

eider duck (noun)	eider (noun)
duck of the Northern Hemisphere much valued for the fine soft down of the females	duck of the Northern Hemisphere much valued for the fine soft down of the females