black-backed gull vs great black-backed gull

black-backed gull and great black-backed gull both are nouns.

Word	Noun	Adjective	Verb	Adverb
black-backed gull	Yes	No	No	No
great black-backed gull	Yes	No	No	No

As nouns, black-backed gull and great black-backed gull are synonyms defined as:

black-backed gull and great black-backed gull: white gull having a black back and wings

Other synonyms of black-backed gull include cob, Larus marinus.

black-backed gull (noun)	great black-backed gull (noun)
white gull having a black back and wings	white gull having a black back and wings