The best and most successful designs work together as a system, and none of us puts together the best thing for ourselves and our situation first time around. Just hoping I help you avoid any of the more obvious mistakes. (and then, when we get it **mostly** right, our situation changes).
You can attach "skirting" to the sides of your tractor, that you can temporarily tie up while moving, then drop back on the ground to deter predators from digging and also encourage your birds to stay in. Just make sure its something fluid/flexible, more like chain link fencing than hardware cloth or chicken wire. Its not perfect - nothing is - and it adds weight (a lot), but it is ""doable"" and readily available material.
You can attach "skirting" to the sides of your tractor, that you can temporarily tie up while moving, then drop back on the ground to deter predators from digging and also encourage your birds to stay in. Just make sure its something fluid/flexible, more like chain link fencing than hardware cloth or chicken wire. Its not perfect - nothing is - and it adds weight (a lot), but it is ""doable"" and readily available material.