reedbird vs Old World warbler

reedbird and Old World warbler both are nouns.

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

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

reedbird: small European warbler that breeds among reeds and wedges and winters in Africa
Old World warbler: small active brownish or greyish Old World birds

Other hypernyms of reedbird include true warbler.

reedbird (noun)	Old World warbler (noun)
migratory American songbird	small active brownish or greyish Old World birds
small European warbler that breeds among reeds and wedges and winters in Africa