pelagic bird vs seabird

pelagic bird and seabird both are nouns.

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

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

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

Other hypernyms of pelagic bird include sea bird, seafowl.

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