bird vs aquatic bird

bird and aquatic bird both are nouns.

As nouns, aquatic bird is a hyponym of bird; that is, aquatic 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
aquatic bird: wading and swimming and diving birds of either fresh or salt water

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, 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, twitterer.

bird (noun)	aquatic bird (noun)
warm-blooded egg-laying vertebrates characterized by feathers and forelimbs modified as wings	wading and swimming and diving birds of either fresh or salt water
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)	aquatic bird (verb)
watch and study birds in their natural habitat