We have 15 birds that completely free range, 23 birds that are being introduced to free range (They are locked at night, but spend the day with their coop door open when we are working in the pastures/barn), and 10 birds in the basement for the winter as they are only 3 weeks old. We also have a mixed flock- Sex links, silkies, phoenix and Sumatran.
We have just under 80 acres; some wooded most of it crop land. Birds are free to come and go as they please. We haven't had *knock on wood* any predator deaths thus far. I'm thinking most of it is due to the fact that the chickens tend to take cover with the horses/goats when there is anything lurking. After the birds got past the 12 week mark we have had 1 death and that was due to a silky rooster who went to perch on the horse water bin, and must have fell in through the night and couldn't get out.
We have found that most of the birds stay within running distance of the barn/horse pastures or near the farm equipment that they can run under for cover. We do have one rooster who will take his 6 hens up by the peacock/duck pen and to forage under the pine trees up there.
My fiance's aunt tried to free range her birds and something was picking them off. She is right next to the property our birds are on (and the birds would all co-exist together, ours coming home at night and hers going home at night). I can only think that this was because her birds other then the coop didn't have the shelter/protection of the other animals (horses), so something was taking them.