greater scaup vs scaup duck

greater scaup and scaup duck both are nouns.

Word	Noun	Adjective	Verb	Adverb
greater scaup	Yes	No	No	No
scaup duck	Yes	No	No	No

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

greater scaup: large scaup of North America having a greenish iridescence on the head of the male
scaup duck: diving ducks of North America having a bluish-grey bill

Other hypernyms of greater scaup include bluebill, broadbill, scaup.

greater scaup (noun)	scaup duck (noun)
large scaup of North America having a greenish iridescence on the head of the male	diving ducks of North America having a bluish-grey bill