I currently have 6 chickens in a 8 x 20 pen with nesting boxes and a coop. I have a large yard and I want them to be able to come out of the pen an get some open air. I have a 6 foot fence around the yard and plenty of space for the. My concern is them going over the fence. They just turned a year old and do not know anything but the pen and coop they live in. What do I have to worry about if I let them out into the yard???