bird vs ratite bird

bird and ratite bird both are nouns.

As nouns, ratite bird is a hyponym of bird; that is, ratite bird is a word with a more specific, narrower meaning than bird:

bird: warm-blooded egg-laying vertebrates characterized by feathers and forelimbs modified as wings
ratite bird: flightless birds having flat breastbones lacking a keel for attachment of flight muscles: ostriches; cassowaries; emus; moas; rheas; kiwis; elephant birds

Other hyponyms of bird include dickey-bird, dickeybird, dicky-bird, dickybird, cock, hen, nester, night bird, bird of passage, protoavis, Archaeopteryx lithographica, archaeopteryx, archeopteryx, Sinornis, Ibero-mesornis, archaeornis, ratite, flightless bird, carinate, carinate bird, flying bird, passeriform bird, passerine, nonpasserine bird, raptor, raptorial bird, bird of prey, gallinacean, gallinaceous bird, parrot, cuculiform bird, coraciiform bird, apodiform bird, caprimulgiform bird, piciform bird, trogon, aquatic bird, twitterer.

bird (noun)	ratite bird (noun)
warm-blooded egg-laying vertebrates characterized by feathers and forelimbs modified as wings	flightless birds having flat breastbones lacking a keel for attachment of flight muscles: ostriches; cassowaries; emus; moas; rheas; kiwis; elephant birds
the flesh of a bird or fowl (wild or domestic) used as food
informal terms for a (young) woman
a cry or noise made to express displeasure or contempt
badminton equipment consisting of a ball of cork or rubber with a crown of feathers

bird (verb)	ratite bird (verb)
watch and study birds in their natural habitat