An old farmer here told me if we ever had foxes that outsmarted the electric fencing and killed a bird, to wrap the bird up in the hot wire (with it off of course for your safety) and crank the conductor up to the highest setting possible. Either you'll fry his tail, or he'll get a nasty enough shock that he won't come back. I know this works for family dogs that decide to kill chickens; my parents had to do this with a prize hen years ago with our dog - and he NEVER went near the birds again. I don't know that this will work for your fox, but it may be an option to try if you're still having problems. I hope you shoot the rascal and turn him into a nice fur. Foxes are gorgeous creatures, for sure - but they are not friends to our birds.
Our biggest issue here where we live is coyotes and hawks. Since the coyotes only come out at night around here, and the birds will be cooped at night - I'm thinking this is a non-issue. It's the hawks during the daylight hours when our birds will be out in their respective pastures that worry me. A neighbor a few miles down the road took video of a hawk actually pulling LOOSE the poultry fencing that covered his girls' run so he could get in. Once he got inside, he decimated about 1/4 of his flock before the guy was able to get off a good shot with a .22 to take him out. That hawk now graces his wall in his den.
