Mergus merganser americanus vs American merganser

Mergus merganser americanus and American merganser both are nouns.

Word	Noun	Adjective	Verb	Adverb
Mergus merganser americanus	Yes	No	No	No
American merganser	Yes	No	No	No

As nouns, Mergus merganser americanus and American merganser are synonyms defined as:

Mergus merganser americanus and American merganser: common North American diving duck considered a variety of the European goosander

Mergus merganser americanus (noun)	American merganser (noun)
common North American diving duck considered a variety of the European goosander	common North American diving duck considered a variety of the European goosander