What has happened to me is just as HF said.
I lost 4 chooks last year in one go, we were in the garden having lunch while the chooks were in a field behind the barn. The cries were really upsetting to hear and the fear in the rest of the chickens as they just ran in all directions was horrible. We eventually rounded them all up and I had lost 4 including the main man. We told one of our neighbours who called in Chasse who came over that afternoon , they didn't catch anything then but apparently did later. No more problems for the rest of the year.
This year I have lost 4 also, one at a time, though and quite close to the house. I also had one attack in the wood, Smokey was badly cut all up her back, but she survived. My cockerel became extremely over protective of the girls and started attacking me and my youngest son, so he had to go.
I kept them in their run, and one lunchtime heard a cuffuffle in the henhouse to see a fox sitting about 2 metres from the wire! It just stared at me and only ran when my JRT took chase.
I bought an electric fence and have had no more trouble, but I used to carry them to their pen everyday!
This summer I will attach the electric pen to their run and put in a little door to give them more space.
I managed to buy a solar powered fence on ebay, which was much cheaper than buying new!
Having seen the way the attacks affected the chickens, I am happy for them not to free range plus the fact that my son is totally against another cockerel, which I think they need for a little protection and to keep them in order!
I am going to give them some free roaming next year, probably for few hours when I can be near them,but otherwise they are in their pen!