Is it possible to hang some bird netting up around the fence?
We did that around our coop and run and our chickens range area of our yard to keep our chickens from flying out into the other part of our yard. Our dogs are not chicken friendly, and their are other predators in our woods too so keeping them in their area has been wonderful.
We spent 30$ at a hardware store for the netting.
The added benefit has been squirrels who used to steal their way into the run and much out on Layer food have stopped sneaking in and even though the netting might need to be repaired occasionally because of limbs falling, It has been given us peace of mind that our girls food isn't disappearing and they are safer too.
Good luck with the duck problem.