I have been raising ducks/geese/quail etc for probably 15 years now. I have had this happen on at least 3 occasions to my birds. At night something would get into the cage/pen and rip off legs mostly. I have had this happen before where a duck would still be alive when I got up in the morning but his entire leg would be ripped off. The rest of the bird was intact. I have also had 2 occasions where only the head has been taken off of my ducks and the animal left the rest of the bird there not to eat? Would could possibly be doing this?