Well, they don't realize that they are tunneling out, but they'be been scratching the dirt under the inside edges of their run. I had built the run with buried 2x6 boards around the perimeter of the run and fixed the chicken wire to the boards. Over time the chickens have scratched quite large cavities out under the inside edges of the boards. If a predator was to dig down on the outside of the run, they'd easily make a hole and probably get in. I've got to take care of this fairly quickly and have been thinking about adding extra wire, or boarding up below the boards, or even forming up the inside and pouring concrete in from the outside into the cavities. Anyone else have a similar issue and experiences/suggestions/bright ideas they can share?