I think hawks are pretty smart, especially when it comes to a juicy dinner. Remember seeing a video (BYC or elsewhere) where a hawk found a way to get into the chickens and made itself a feast. Having said that, I have a small completely enclosed run for my 5 girls, but they spend a good part of the day in the fenced-in backyard. Their favorite space is under a row of pine trees aside our fence and in a small wooded area near the pines. We have red hawks, etc., so I think I'm dancin' with the devil a little here. This spring, we're expanding with a 2nd coop and much larger run (plus more chickens), and that will have a hardware cloth ceiling.