diving petrel vs pelagic bird

diving petrel and pelagic bird both are nouns.

Word	Noun	Adjective	Verb	Adverb
diving petrel	Yes	No	No	No
pelagic bird	Yes	No	No	No

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

diving petrel: any of several small diving birds of Southern Hemisphere seas; somewhat resemble auks
pelagic bird: bird of the open seas

Other hypernyms of diving petrel include oceanic bird.

diving petrel (noun)	pelagic bird (noun)
any of several small diving birds of Southern Hemisphere seas; somewhat resemble auks	bird of the open seas