hooded merganser vs fish duck

hooded merganser and fish duck both are nouns.

Word	Noun	Adjective	Verb	Adverb
hooded merganser	Yes	No	No	No
fish duck	Yes	No	No	No

As nouns, fish duck is a hypernym of hooded merganser; that is, fish duck is a word with a broader meaning than hooded merganser:

hooded merganser: small North American duck with a high circular crest on the male's head
fish duck: large crested fish-eating diving duck having a slender hooked bill with serrated edges

Other hypernyms of hooded merganser include merganser, sawbill, sheldrake.

hooded merganser (noun)	fish duck (noun)
small North American duck with a high circular crest on the male's head	large crested fish-eating diving duck having a slender hooked bill with serrated edges