Old World warbler vs true warbler

Old World warbler and true warbler both are nouns.

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

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

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

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