hooded merganser vs merganser

hooded merganser and merganser both are nouns.

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

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

hooded merganser: small North American duck with a high circular crest on the male's head
merganser: large crested fish-eating diving duck having a slender hooked bill with serrated edges

Other hypernyms of hooded merganser include fish duck, sawbill, sheldrake.

hooded merganser (noun)	merganser (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