sea bird vs aquatic bird

sea bird and aquatic bird both are nouns.

Word	Noun	Adjective	Verb	Adverb
sea bird	Yes	No	No	No
aquatic bird	Yes	No	No	No

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

sea bird: a bird that frequents coastal waters and the open ocean: gulls, pelicans, gannets, cormorants, albatrosses, petrels, etc.
aquatic bird: wading and swimming and diving birds of either fresh or salt water

sea bird (noun)	aquatic bird (noun)
a bird that frequents coastal waters and the open ocean: gulls, pelicans, gannets, cormorants, albatrosses, petrels, etc.	wading and swimming and diving birds of either fresh or salt water