barn owl vs bird of night

barn owl and bird of night both are nouns.

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

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

barn owl: mottled buff and white owl often inhabiting barns and other structures; important in rodent control
bird of night: nocturnal bird of prey with hawk-like beak and claws and large head with front-facing eyes

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

barn owl (noun)	bird of night (noun)
mottled buff and white owl often inhabiting barns and other structures; important in rodent control	nocturnal bird of prey with hawk-like beak and claws and large head with front-facing eyes