American merganser vs fish duck

American merganser and fish duck both are nouns.

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

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

American merganser: common North American diving duck considered a variety of the European goosander
fish duck: large crested fish-eating diving duck having a slender hooked bill with serrated edges

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

American merganser (noun)	fish duck (noun)
common North American diving duck considered a variety of the European goosander	large crested fish-eating diving duck having a slender hooked bill with serrated edges