We have three pet hens, and what started out as letting them out a few times a week for a few hours has turned into them having to be let out all day, every day, or they "scream" in their coop and run. Since I live in a small city, the noise was too much. They are happy and healthy, and it is the cutest thing the way they follow our family all around the yard, but I do fear one day something will come and possibly get all three of them, since there are only three to get, and we have no rooster or dogs outside.
In our favor, we have lots of natural prey--wild rabbits and rodents, lots of songbirds, etc. There are also much better, more open areas for hunting then my heavily treed and shrubbed yard. We also have tons of crows who live in our yard, and I do believe that has kept the hawks away.
Our next chickens will not be allowed to free range in this way, as it is stressful regarding their safety and requires someone to let them out and lock them up on our vacations (we have an automatic coop door, so if they were just in the run this wouldn't be a problem). I have long planned to build a tractor or pen or something and that will be a way for them to get some forage time in safety. But, it is pretty neat when they come sit on your lap, or hang out with the kids by the sandbox or swing.