a North American game bird (Bonasa umbellus) with neck feathers that can be extended into a ruff: also called partridge in the N U.S. and pheasant in the S U.S.
See ruffed grouse in American Heritage Dictionary 4
noun
A chickenlike North American game bird (Bonasa umbellus) having mottled brownish plumage and noted for the drumming sounds the male makes with its wings. Also called birch partridge.