Do you have a way to confine your flock for a while? Once a fox starts, and they are very active in spring on behalf of kits, they simply don't give up. They kill, bury the corpse to ripe and kill again. They try to build up several weeks' worrh of food at a time. We have at least one den on our property so we'll never be able to free-range. I've had foxes hunt the ferals to the point of coming right up to me without noticing I was there. I'm very sorry- this is a huge problem.