Hawks are a tough problem. We lose a few chickens a year to them. I think cover — bushes, picnic tables, shade arbors — really helps the chickens out. I have open fields around my home, and I can go out on the deck and make a red-tailed hawk whistle and the hens all charge over to the garden and hide in the asparagus patch.
— Bryan Welch, farmer and publisher, Mother Earth News
I think it is impossible to raise chickens on range without incurring some loss to predation. My only aerial losses came from night attacks by an owl — we tended to lose white chickens that way. Hawks are thick here in Kansas, but we have lots of trees and shrubs. Our birds head for cover whenever a hawk is overhead, and to my knowledge they have never been attacked by one.
Having written that, I think that pasture pens and the presence of dogs, humans or other large creatures in the area will deter the hawks. I suggest you let your birds range again after about a week to see whether the predator is still around or has moved on.