Hmmmm......fox tracks, once made, last until the snow melts. All day and all night. As I read the tracks, it was pretty obvious that fox did what would be expected.....crawled under and got zapped in the process. Then did the same thing one of our barn cats did.....it ran around all over the place in a panic trying to find a way out, then eventually jumped out......and he never came back. That was 2 years ago. We have had 5 tracking snows so far this year, and each time, I walk the perimeter and no tracks. Nothing. We have a pack of coyotes in the neighborhood.....have heard them multiple times over the past few weeks and close. No more than a few hundred yards or so from the front door. No tracks from them either.
Bottom line is I've never lost a bird to a predator and have never had to shoot one either (skunk that took up residence in the barn doesn't count). I chalk that up to an electric fence that protects them by day and a tight coop that protects them at night. If anyone has a system that produces a better record than that (or can even equal it), I'd like to hear about it.