Dito on "do not drag". Unless the cape was already wet from rain, or dieing in a pond, I would never recommend washing it in the field, unless it can go strait into the freezer! Bacteria growth is the enemy, and bacteria needs moisture.Cold/frozen and dry is your friend.
My advice: cool quickly and freeze! If freezer not immediately available (usually is not), keep it DRY in a cooler with ice, dry ice is best (never in a plastic bag) until drop off at taxidermist or freezer. The absolute NO NO is in a plastic bag in back of your truck in the desert!!!! Alternatively, completely flesh, turn ears, eyes, lips, nose and salt til dry.