owlet vs bird of night

owlet and bird of night both are nouns.

Word	Noun	Adjective	Verb	Adverb
owlet	Yes	No	No	No
bird of night	Yes	No	No	No

As nouns, bird of night is a hypernym of owlet; that is, bird of night is a word with a broader meaning than owlet:

owlet: young owl
bird of night: nocturnal bird of prey with hawk-like beak and claws and large head with front-facing eyes

Other hypernyms of owlet include bird of Minerva, hooter, owl.

owlet (noun)	bird of night (noun)
young owl	nocturnal bird of prey with hawk-like beak and claws and large head with front-facing eyes