Hi,your best bet wold be erect something to stop the fox from getting to your birds in the first place.A fence preferably electrified will help.
It doesn't matter how many foxes you kill,there will be another one along sooner or later.And one thing you an be sure of with a fox is,, if it can get to your birds sooner or later it will
Human and dog pee is just an old wives tale it doesn't work.
Prevention is always better that cure,but that dosn't mean I wouldn't be reaching for my 12 bore.
Graham.