Chickens can get out of that pretty easy, or at least mine can.
I have the 48" from Premier 1. I admit that Chipotle, the California White, can fly out and back in at will. But the others only fly over when powerfully motivated -- usually when startled vertical -- and they all stay close to the pen where the rest of the flock is. I'm fairly sure that the Light Brahma couldn't get out no matter how powerfully motivated she might be. I haven't seen her get over 18" off the ground since she was 6 months old.