Mergus albellus vs merganser

Mergus albellus and merganser both are nouns.

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

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

Mergus albellus: 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 Mergus albellus include fish duck, sawbill, sheldrake.

Mergus albellus (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