The fox will keep coming back because it has killed. You will continue to loose birds as long as you free range. Lessons learned the hard way. Now I have electric wires around my coops and pens. Once a predator gets zapped it will respect the hot wires and so will your dog and cat. I have accidentally touched my wires and it hurts, but this is the idea I want whatever touches the wires to hurt to get the message that whatever is on the other side of the hot wires isn't worth getting zapped for. Good luck with your birds and have fun...
https://www.plamondon.com/wp/faq-simple-electric-fences-chickens/
https://www.plamondon.com/wp/faq-simple-electric-fences-chickens/