Foxes are everywhere, even if you don't see them very often.
They will check out your poultry/duck pens most evenings/mornings on their way by, and if there is a chance to kill something they will take it.
Electric netting can be useful, but you must take care that it doesn't snag on other fencing or vegetation, as that will take the power down and allow the fox to raid. I had some beautiful laying ducks killed one Christmas night and, because the electric net had blown against the barbed wire stock fence, causing it to short, the fox was able to gnaw through a couple of strands and pull the headless drake through the net.........the ten ducks were killed, their heads removed, and left in the run
I keep quite a lot of poultry and have had some sad losses over the years.............I don't keep ducks or turkeys now as the foxes cleaned me out, even in broad daylight.
I used to try to live with the foxes and suffer the losses, but when you lose your best stock birds time after time it's hard to find anything endearing about them. I now operate a line of snares at various times of the year, for foxes, and finally have some respite from their predations. Having killed a pair just a couple of weeks ago, I will probably be fox free-ish for the best part of this summer................it's a bit late for another pair to move in, as most vixens will be settled in cubbing territories by now......until the neighbouring cubs start raking.
Foxes use the farm track past the house as a thoroughfare, and I've even had them in the garden, which is very small, scattering the hens. I would rather leave them alone but in the past eight years I've had to shoot them within yards of the house/outlying hen runs, at all times of the day and night.
My neighbouring farmer is very diligent, and there are some lampers about in the winter too, so the numbers are much more acceptable now.
If you do plan to keep hens in an electric fence run, I'd advise you to check the fence at least every morning, before you let the birds out, for power and freedom from entanglement.