fish duck vs sheldrake

fish duck and sheldrake both are nouns.

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

As nouns, fish duck and sheldrake are synonyms defined as:

fish duck and sheldrake: large crested fish-eating diving duck having a slender hooked bill with serrated edges

Other synonyms of fish duck include merganser, sawbill.

fish duck (noun)	sheldrake (noun)
large crested fish-eating diving duck having a slender hooked bill with serrated edges	Old World gooselike duck slightly larger than a mallard with variegated mostly black-and-white plumage and a red bill
	large crested fish-eating diving duck having a slender hooked bill with serrated edges