coastal diving bird vs sea bird

coastal diving bird and sea bird both are nouns.

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

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

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

Other hypernyms of coastal diving bird include seabird, seafowl.

coastal diving bird (noun)	sea bird (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.