pelagic bird vs sea bird

pelagic bird and sea bird both are nouns.

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

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

pelagic bird: bird of the open seas
sea bird: a bird that frequents coastal waters and the open ocean: gulls, pelicans, gannets, cormorants, albatrosses, petrels, etc.

Other hypernyms of pelagic bird include seabird, seafowl.

pelagic bird (noun)	sea bird (noun)
bird of the open seas	a bird that frequents coastal waters and the open ocean: gulls, pelicans, gannets, cormorants, albatrosses, petrels, etc.