scoter vs sea duck

scoter and sea duck both are nouns.

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

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

scoter: large black diving duck of northern parts of the Northern Hemisphere
sea duck: any of various large diving ducks found along the seacoast: eider, scoter, merganser

scoter (noun)	sea duck (noun)
large black diving duck of northern parts of the Northern Hemisphere	any of various large diving ducks found along the seacoast: eider, scoter, merganser