grey birch vs gray birch

grey birch and gray birch both are nouns.

Word	Noun	Adjective	Verb	Adverb
grey birch	Yes	No	No	No
gray birch	Yes	No	No	No

As nouns, grey birch and gray birch are synonyms defined as:

grey birch and gray birch: medium-sized birch of eastern North America having white or pale grey bark and valueless wood; occurs often as a second-growth forest tree

Other synonyms of grey birch include American gray birch, American grey birch, Betula populifolia.

grey birch (noun)	gray birch (noun)
medium-sized birch of eastern North America having white or pale grey bark and valueless wood; occurs often as a second-growth forest tree	medium-sized birch of eastern North America having white or pale grey bark and valueless wood; occurs often as a second-growth forest tree