Regulus regulus vs kinglet

Regulus regulus and kinglet both are nouns.

Word	Noun	Adjective	Verb	Adverb
Regulus regulus	Yes	No	No	No
kinglet	Yes	No	No	No

As nouns, kinglet is a hypernym of Regulus regulus; that is, kinglet is a word with a broader meaning than Regulus regulus:

Regulus regulus: European kinglet with a black-bordered yellow crown patch
kinglet: small birds resembling warblers but having some of the habits of titmice

Regulus regulus (noun)	kinglet (noun)
European kinglet with a black-bordered yellow crown patch	small birds resembling warblers but having some of the habits of titmice