rhea vs flightless bird

rhea and flightless bird both are nouns.

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

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

rhea: larger of two tall fast-running flightless birds similar to ostriches but three-toed; found from Brazil to Patagonia
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 rhea include ratite, ratite bird.

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

rhea: smaller of two tall fast-running flightless birds similar to ostriches but three-toed; found from Peru to Strait of Magellan
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 rhea include ratite, ratite bird.

rhea (noun)	flightless bird (noun)
larger of two tall fast-running flightless birds similar to ostriches but three-toed; found from Brazil to Patagonia	flightless birds having flat breastbones lacking a keel for attachment of flight muscles: ostriches, cassowaries, emus, moas, rheas, kiwis, elephant birds
smaller of two tall fast-running flightless birds similar to ostriches but three-toed; found from Peru to Strait of Magellan