Allowing chickens to roam free has many benefits, the main ones being access to a diverse and healthy diet and mental stimulation. However, when they are out of their secure coop/run, you are exposing your chickens to the chance that a predator may attack or even kill one. Hawks, foxes, bobcats, and domestic dogs are some of the more common daytime predators, but your risk really depends on your location and surroundings. Some people prefer to supervise their flocks when they are out, but even this will not prevent all losses. Just understand you are taking a risk.
That being said, all my birds (including the tiny Seramas) roam free during the day on a fenced property. Every morning, they are anxious to be let out, and only head back to the coop to lay. They return to the coop before dark with out my help, or I can call them in early if needed (with scratch). Once they are out, it is obvious that they prefer it, and I am okay with the risk, given that we are in a fairly safe area.
The farm chickens we had when I was young had 33 acres on which to roam, but tended to stay near the barn or their coop, limiting themselves to ranging on about 2-3 acres. Occasionally a hen would disappear and show up 3 weeks later with a brood of chicks. It is a very good idea to keep a rooster with roaming hens. He will devote himself to warning and protecting them.
