We have tried to raise all of our poultry on pasture. We had 7 Buff ducks in a hoop house style "duck tractor" made on a 2x4 frame with cattle panels bent in an arc. Hardware cloth around the bottom half, and 14 gauge 2x3 fence over the top half of the sides, and hardware cloth on the front and back.
This past weekend we got hit by a predator. We lost 6 of the 7 ducks, including a broody hen in her last week of sitting on 6 eggs.
I took some time to try and figure out how it got in. I am fairly certain now that it was a fox. It might have been a raccoon, but the droppings are a bit more consistent with a fox. Our pasture is not even, and the ground is rough. From what I can piece together, it appears the predator was able to wedge under a small gap between the ground and the sled and dig it's way in just a hair more. We had triple digit heat for about 10 days in a row, and that night I had decided to go spray the ducks down with a hose. I am thinking in doing so, I softened the ground around the tractor a bit as well, making it easier to dig in.
I believe someone else also posted this, but nothing is 100% predator proof. Most of the things I have built, have been done with predators in mind. A lot of my farm friends tell me I overbuild. This experience has made me think more about building portable coops attached to larger runs (which I do have a couple of already), to give me something to lock animals up in at night with solid walls. I am also thinking more about the electric fence option.