smew vs merganser

smew and merganser both are nouns.

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

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

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

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

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