Old World warbler vs warbler

Old World warbler and warbler both are nouns.

Word	Noun	Adjective	Verb	Adverb
Old World warbler	Yes	No	No	No
warbler	Yes	No	No	No

As nouns, warbler is a hypernym of Old World warbler; that is, warbler is a word with a broader meaning than Old World warbler:

Old World warbler (noun)	warbler (noun)
small active brownish or greyish Old World birds	a small active songbird
	a singer; usually a singer who adds embellishments to the song