redhead vs Aythya americana

redhead and Aythya americana both are nouns.

Word	Noun	Adjective	Verb	Adverb
redhead	Yes	No	No	No
Aythya americana	Yes	No	No	No

As nouns, redhead and Aythya americana are synonyms defined as:

redhead and Aythya americana: North American diving duck with a grey-and-black body and reddish-brown head

redhead (noun)	Aythya americana (noun)
black-and-white North American woodpecker having a red head and neck	North American diving duck with a grey-and-black body and reddish-brown head
North American diving duck with a grey-and-black body and reddish-brown head
someone who has red hair