pelagic bird vs petrel

pelagic bird and petrel both are nouns.

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

As nouns, petrel is a hyponym of pelagic bird; that is, petrel is a word with a more specific, narrower meaning than pelagic bird:

pelagic bird: bird of the open seas
petrel: relatively small long-winged tube-nosed bird that flies far from land

Other hyponyms of pelagic bird include procellariiform seabird, albatross, mollymawk, diving petrel.

pelagic bird (noun)	petrel (noun)
bird of the open seas	relatively small long-winged tube-nosed bird that flies far from land