great grey owl vs bird of night

great grey owl and bird of night both are nouns.

Word	Noun	Adjective	Verb	Adverb
great grey owl	Yes	No	No	No
bird of night	Yes	No	No	No

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

great grey owl: large dish-faced owl of northern North America and western Eurasia
bird of night: nocturnal bird of prey with hawk-like beak and claws and large head with front-facing eyes

Other hypernyms of great grey owl include bird of Minerva, hooter, owl.

great grey owl (noun)	bird of night (noun)
large dish-faced owl of northern North America and western Eurasia	nocturnal bird of prey with hawk-like beak and claws and large head with front-facing eyes