I've heard it several time how the hen house has been raided in "city" dwellings. You may not see the predators, but they are out there. That's why I'm taking no chances. You get some chickens and predators just appear from no where. I'm not just talking about feral cats and stray dogs, but there are more. A fox can easily scale a 6-foot wall with a chicken in its mouth. I don't know what other critters are out there, whether it be raccoons, possums etc. Strange things can show up once you start having chickens. It would break my heart to walk out to my back yard and see a massacre with feathers all over the place.If they are kept in areas where predators can get to them, eventually they will.
I figure 160 square feet of outdoor run space should be enough for at least a half dozen birds, and that's excluding the Chicken TV deck and the 6x6 coop.