eider vs eider duck

eider and eider duck both are nouns.

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

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

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

eider (noun)	eider duck (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