Show a picture of where fox penetrates fencing material. You had to get very close to it to see blood from either fox or chicken on fox. In most instances, and I have seen many, attacks by fox are relatively bloodless unless fox has to pull live victim through wire and victim is flinging blood...
Blood almost certainly not from fox, rather blood is from bird. Predators very seldom want to damage themselves for a meal. When I had fox issue, it had to partially dismember bird to get it though fencing.
Fox will stop if it is not catching birds. It can not afford too many futile efforts owing to energy constraints. I have had to live with foxes and it can be done even if fox not removed. I place obstacles around where fox might dig. Does not take much to stop digging. Again, fox issue of...
Trap fox using bird killed as bait.
Smaller faster dog would be good for next fox. Big dogs are not fast enough. Fox small with long tail simply too maneuverable.