sheldrake vs fish duck

sheldrake and fish duck both are nouns.

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

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

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

Other synonyms of sheldrake include merganser, sawbill.

sheldrake (noun)	fish duck (noun)
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
large crested fish-eating diving duck having a slender hooked bill with serrated edges