My backyard is surrounded by a 6' privacy fence. I've had a flock back there for three years. When the birds are about six months old they can clear it, but as they grow older they get heavier and quit trying. With my first flock there were several weeks when I was continually retrieving the same two or three birds who had figured it out. I've only lost one to a neighbor dog over the past three years. I suspect that since it is a solid fence most have no urge to go over. If they could see through it and realize they could be on the other side, it could be more of a problem.
I let them run back there all day, they put themselves up at night and come on out of the coop in morning. The fence is tight though, no critters get in and the surrounding neighbor dogs keep most critters out of the area anyway.