Chickens love to explore. Mine will escape when given the chance and since my neighbors have a problem with chicken poo on their neatly manicured lawns I have to be the warden. When they were younger gals and not quite so plump they could easily fly over the four foot fence surrounding our backyard. It is truly amazing how well a Rhode Island Red on the lamb can play chameleon in a residential neighborhood. It wasn't long before I grew tired of chasing down escaped inmates. That's when the scissors came out and the girls got their first "haircuts". Clipping their right wing feathers put the brakes on unauthorized flights and made for happier neighbors and less stress for me.
I've also had to reinforce my fence line several times as my chickens will dig along the edge with such energy it would put Rover to shame. In several areas planting a line of plumbago along the fence has provided a barrier and put an end to our real life "Chicken Run" drama. Its been quite a while now since anyone has made a break for it and I think they've realized all attempts are now futile. Perhaps they've learned that the grass is not any greener on the other side of the fence even if its cut a little neater and prettier over there.