Mergus merganser vs sawbill

Mergus merganser and sawbill both are nouns.

Word	Noun	Adjective	Verb	Adverb
Mergus merganser	Yes	No	No	No
sawbill	Yes	No	No	No

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

Mergus merganser: common merganser of Europe and North America
sawbill: large crested fish-eating diving duck having a slender hooked bill with serrated edges

Other hypernyms of Mergus merganser include fish duck, merganser, sheldrake.

Mergus merganser (noun)	sawbill (noun)
common merganser of Europe and North America	large crested fish-eating diving duck having a slender hooked bill with serrated edges