Mergus albellus vs sawbill

Mergus albellus and sawbill both are nouns.

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

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

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

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

Mergus albellus (noun)	sawbill (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