little owl vs bird of night

little owl and bird of night both are nouns.

Word	Noun	Adjective	Verb	Adverb
little owl	Yes	No	No	No
bird of night	Yes	No	No	No

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

little owl: small European owl
bird of night: nocturnal bird of prey with hawk-like beak and claws and large head with front-facing eyes

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

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