smew vs sheldrake

smew and sheldrake both are nouns.

Word	Noun	Adjective	Verb	Adverb
smew	Yes	No	No	No
sheldrake	Yes	No	No	No

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

smew: smallest merganser and most expert diver; found in northern Eurasia
sheldrake: large crested fish-eating diving duck having a slender hooked bill with serrated edges

Other hypernyms of smew include fish duck, merganser, sawbill.

smew (noun)	sheldrake (noun)
smallest merganser and most expert diver; found in northern Eurasia	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