coastal diving bird vs seafowl

coastal diving bird and seafowl both are nouns.

Word	Noun	Adjective	Verb	Adverb
coastal diving bird	Yes	No	No	No
seafowl	Yes	No	No	No

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

coastal diving bird: gull family; skimmer family; jaeger family; auk family
seafowl: a bird that frequents coastal waters and the open ocean: gulls, pelicans, gannets, cormorants, albatrosses, petrels, etc.

Other hypernyms of coastal diving bird include sea bird, seabird.

coastal diving bird (noun)	seafowl (noun)
gull family; skimmer family; jaeger family; auk family	a bird that frequents coastal waters and the open ocean: gulls, pelicans, gannets, cormorants, albatrosses, petrels, etc.