true warbler vs Old World warbler

true warbler and Old World warbler both are nouns.

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

As nouns, true warbler and Old World warbler are synonyms defined as:

true warbler and Old World warbler: small active brownish or greyish Old World birds

true warbler (noun)	Old World warbler (noun)
small active brownish or greyish Old World birds	small active brownish or greyish Old World birds