I really like a prior poster's suggestion of a fast growing semi-seasonal vine which might be supported by wire or the like to create a natural "roof" . Of the two suggestions, I'd favor the english ivy (which I personally love). I have this thought in the back of my skull, perhaps wrong, that the other quick grower isn't good for chickens to eat. and I know birds don't tend to eat things bad for them for long, or in any quantity, but I'd still shy away if a better alternative was readilt available.