bird vs Ibero-mesornis

bird and Ibero-mesornis both are nouns.

As nouns, Ibero-mesornis is a hyponym of bird; that is, Ibero-mesornis 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
Ibero-mesornis: sparrow-sized fossil bird of the Cretaceous period having a vestigial tail; found in Spain; considered possibly the third most primitive of all 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, archaeornis, ratite, ratite bird, 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)	Ibero-mesornis (noun)
warm-blooded egg-laying vertebrates characterized by feathers and forelimbs modified as wings	sparrow-sized fossil bird of the Cretaceous period having a vestigial tail; found in Spain; considered possibly the third most primitive of all 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)	Ibero-mesornis (verb)
watch and study birds in their natural habitat