Origin of the "white feather as a symbol of cowardice"

I’m researching the origins of the white feather as a symbol of cowardice. The earliest reference I can find is in Francis Grose’s 1785 Dictionary of the Vulgar Tongue. Does anyone know of any earlier occurrences?

The OED uses Grose's dictionary as its first source and adds

1. A white feather as a symbol of cowardice. [...]With allusion to the fact that a white feather in a game bird's tail was considered a mark of inferior breeding.

1785 F. Grose Classical Dict. Vulgar Tongue White feather, he has a white feather, he is a coward, an allusion to a game cock, where having a white feather, is a proof he is not of the true game breed.

Grose's example, game cock/breed is a use of "game" in the hunting sense and OED game, adj. 1. Of a person or animal: full of pluck, spirit, or fight; spirited, plucky; intrepid. Also applied to actions, attributes, etc.