I got ya. I have a cornish cross, known for being lethargic, that would come flying by at least at shoulder high every day to beat me out to the feed can. I've seen chickens roosting 20 ft up in trees, literally 10' pole, 8 foot reach, and having to drive the pick up underneath to stand in the bed to catch them. You can always trim their primary feathers, that would easily keep them in 8' when they get older. Just leave enough they can steady themselves on the roost. If they are exposed to any predators you don't want to do this. For getting out of the pen, from my experience it isn't flighty that causes it, it's "hey there's something good to eat over there".