aepyornis vs flightless bird

aepyornis and flightless bird both are nouns.

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

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

aepyornis: huge (to 9 ft.) extinct flightless bird of Madagascar
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 aepyornis include ratite, ratite bird.

aepyornis (noun)	flightless bird (noun)
huge (to 9 ft.) extinct flightless bird of Madagascar	flightless birds having flat breastbones lacking a keel for attachment of flight muscles: ostriches, cassowaries, emus, moas, rheas, kiwis, elephant birds