Trees help break up a hawk's line of sight while they're soaring around looking for food, if nothing else. I have several with low hanging branches in my backyard, and while I've seen plenty of hawks, I've never lost a bird to one. My chickens don't free range all the time, but when they do, I'm generally not supervising (though I am usually in earshot). Sometimes I put my dog out there to keep an eye on them (Great Pyrenees/Anatolian Shepherd) if I'm feeling nervous. There always seems to be one or two chickens with their eye on the sky, and they duck and run under the deck or into the bushes when they see a shadow.
I've planted an Osage Orange screen to shade my run (and muffle that glorious egg song). They grow fast, and once they get tall and dense enough, ain't nothing coming through 'em, and I'll be keeping them trimmed around the base so both my chickens and I can safely pass under. Closer to the run I'm slowly adding Barberry bushes. The chickens don't bother them, and I figure they'll discourage anything from trying to get through the fence.