I have had a few fox attack my chickens over the years. The one thing that I can say is if one of your chickens gets attacked and you did not have your gun handy go get it. The fox is in my case continued to come back two or three more times allowing me to finally get off a few shots killing them. They very rarely would attack a bird and not come back for the rest.
I have shot three so far so if you’re keeping count it’s Fox 2. Chicken protector 3 with the win.