As others have stated a covered run is a must if you want to avoid this problem 100%.
At my last house, it was set up so you could see most of the yard. So, when a hawk was sighted,
out went a rottwieler. As she is like shooting a bullet out the backdoor. The chickens would run to her
for saftey. If I had chickens on both sides of the yard and a hawk, out went one of my cocker spaniels
to help the rottwieler out. It seemed to make the hawks think twice.
The only problem with that is the dogs were not out all the time. I did loose a chicken in the moving
transition to a hawk. But, there were people living there with NO dogs. So the yard was not as safe
as it had been before.
If you choose to free range, I think you need to realize things happen. And at least your chickens
lived the life of a happy chicken.
p.s. it took two of us (dog trainers) to get the rottwieler 100% chicken safe around chickens and a years time. Now all I have to worry about is one getting in her way when she is after a squirrel. LOLOL that is one great dog! Cuz if she hits something
its like getting hit by a frieght train.