nighthawk vs nightjar

nighthawk and nightjar both are nouns.

Word	Noun	Adjective	Verb	Adverb
nighthawk	Yes	No	No	No
nightjar	Yes	No	No	No

As nouns, nightjar is a hypernym of nighthawk; that is, nightjar is a word with a broader meaning than nighthawk:

nighthawk: mainly nocturnal North American goatsucker
nightjar: mainly crepuscular or nocturnal nonpasserine birds with mottled greyish-brown plumage and large eyes; feed on insects

Other hypernyms of nighthawk include caprimulgid, goatsucker.

nighthawk (noun)	nightjar (noun)
mainly nocturnal North American goatsucker	mainly crepuscular or nocturnal nonpasserine birds with mottled greyish-brown plumage and large eyes; feed on insects
a person who likes to be active late at night