oceanic bird vs seafowl

oceanic bird and seafowl both are nouns.

Word	Noun	Adjective	Verb	Adverb
oceanic bird	Yes	No	No	No
seafowl	Yes	No	No	No

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

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

Other hypernyms of oceanic bird include sea bird, seabird.

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