If you keep your ducks and chickens together, it could have been the rooster. I doubt it was from another duck trying to get after her since they are all female but stranger things have happened. As far as chickens go, if they see even the tiniest bit of blood or a scab, they'll pick it and can kill the injured bird. The blu kote is a good suggestion as is the neosporin. I sure hope she feels better soon. My ducks like frozen veggies.