hooded sheldrake vs fish duck

hooded sheldrake and fish duck both are nouns.

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

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

hooded sheldrake: 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 sheldrake include merganser, sawbill, sheldrake.

hooded sheldrake (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