“Late season birds are tougher to kill because of their thicker feathers and heavier layers of fat and down.” How many times have you heard that? I’m sorry, but it’s not true.
As fall wears on into winter, wild waterfowl and upland birds have progressively LESS access to food. This is due principally to snow cover. So fat layers and muscles of wild game birds do not get thicker or heavier as fall hunting seasons transcend into winter.