moa vs flightless bird

moa and flightless bird both are nouns.

Word	Noun	Adjective	Verb	Adverb
moa	Yes	No	No	No
flightless bird	Yes	No	No	No

As nouns, flightless bird is a hypernym of moa; that is, flightless bird is a word with a broader meaning than moa:

moa: extinct flightless bird of New Zealand
flightless bird: flightless birds having flat breastbones lacking a keel for attachment of flight muscles: ostriches, cassowaries, emus, moas, rheas, kiwis, elephant birds

Other hypernyms of moa include ratite, ratite bird.

moa (noun)	flightless bird (noun)
extinct flightless bird of New Zealand	flightless birds having flat breastbones lacking a keel for attachment of flight muscles: ostriches, cassowaries, emus, moas, rheas, kiwis, elephant birds