I would leave it but, then if you loose a bird again you will be devastated and feel guilty, I know I would, what about putting up a trap of a dead (bought) chicken or similar and see if a fox comes to take it? Not sure if I could sit around all day watching but I suppose you could put up a webcam!!! A good friend told us his brother has just lost some chickens to a Fox and he had very tight security and as you know, they hunt in the day as well, other than get a marksman on the perimiter fence 24/7 not sure what you can do!!!!!! fingers crossed, your lifestock are safe.