coastal diving bird vs seabird

coastal diving bird and seabird both are nouns.

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

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

coastal diving bird: gull family; skimmer family; jaeger family; auk family
seabird: 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, seafowl.

coastal diving bird (noun)	seabird (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.