What type of hawk is it? Just saying but tree cover will do nothing against the majority of chicken hawks like Red tailed hawks and cooper's hawks, just makes hunting easier. Hawks don't fly around up in the sky diving down on chickens, they will just sit up in a tree and wait for the chickens to come as close as possible then attack. Hawks are very smart and figure things out fast. We had a cooper's hawk many years ago that would pick off bantams from out flock but he did it like it was a game, he would land right next to them instead of just grabbing them and then chase after them and kill them. I also have dog kennels right by the coop, one time a cooper's hawk killed and ate a chicken 3 feet from the kennel and let the dogs watch him the whole time. I saw a cooper's hawk a few months ago miss an attack on one of my birds, he flew extremely low over the kennel and got a stud to jump up and try to grab him, the hawk did not flinch and the stud fell on his butt after missing. In my experience hawks will figure out what is going to harm them and what's not very fast.
Thick small bushes/shrubs to run under will be more helpful than tree cover, they provide cover without providing a hunting spot at the same time. I have not had a successful hawk attack in over 5 years, not sure what is stopping them but I did have 21 cockerels/roosters in the flock this summer. 2 sharp shinned hawks did catch a bird but were unable to kill it.
