I am new on the forum but since no one else has chimed in I thought I would stir up a response for you. At least it will be a bump. I am in California. I live at FOX CENTRAL. I have feral cats at the barn that I feed and supply water for and so over the years have provided many generations of Kit Foxes and Red Foxes with sustenance. I have LOTS of foxes around here especially in the spring, and honestly, I really enjoy them, especially when the babies are running around playing at the barn. I also very much like the reduction in the rodent population. Foxes are territorial and even in the spring you will only see one family of each species hunting in one area. Usually, if any survived, the females from the previous year will help their mother with the current years kits and so you could see "a family" all together in one place but only the mother fox will produce babies. The male offspring from the previous year are driven off and the two year old females leave to stake out their own territory. The daddy only stays around long enough to accomplish "the dead". (adds a whole new meaning to - "that guy is a fox" doesn't it) This has been going on for the 26 years I've lived here, (and a whole lot longer).
Even when I had a simple and raggidy large picket and horse wire enclosure for my chickens I never lost a bird to foxes. Maybe they were getting their fill of cat food but I can't recommend that as it can get expensive. Eventually the raccoons discovered me I had to get more serious. Now my pins are formal and ABSOLUTELY EVERYTHING PROOF. This time of year there is more pressure on the fox family as the kits are here. This is when you will probably see an increase in predation on your birds because of the increased number of mouths to feed in one area. Once the babies are on their own they will disburse. If you trap the mother, or even the daughters, the babies survival is threatened. I know what you are thinking ... BUT, you have to remember, if you have foxes, you are in an area that has foxes. You will always have foxes and there are a couple of endangered species out there that could cost you if you get caught.
The only way to FIX the problem is to ... BUILD A BETTER COOP !!!!
Wow, that two cents turned into a dollar fifty, by now maybe some one else has given you a better answer.