Sorry for coming across so abruptly. From reading your first post, I gathered that you had seen this fox helping himself to your chickens - I figured that's how you knew what it was. It also seemed that it has been going on for quite some time. (I guess I've read too many "something has been eating several of my chickens the past x number of days" posts and read too much into yours.)
My husband used to trap, and fox can be tricky. As I mentioned earlier, a run attached to your coop can be a helpful thing. I have read on here (I have not witnessed it myself, however), that fox can climb, so you may want to run a few strands of electric wire around that run and/or cover it with something sturdy like welded wire instead of tarp or netting. If you have enough room to keep them locked up 24/7 for a few weeks, your fox may get discouraged and move out on its own. Do you know anyone with experience trapping fox? You may need to talk to your state wildlife agency about trapping out of season, though..
I realize a person doesn't have time to sit around and wait for predators to show up. Again, I was surmising you'd seen this animal, and you could be prepared when you were out with your birds to eliminate it if you came upon it.
Good luck - I hope you can either eliminate the problem or encourage it to move on to somewhere else.