Treats, especially meal worms might help them get over it quicker. I only have about a half an acre, but I cut all my fruit trees short and they sort of resemble large umbrellas. The hawks have tried to get my chickens quite a few times, but they know to stay under the trees and the hawks don’t like to go onto the ground, so my chickens just run from cover to cover and hang out underneath the low trees and bushes. When all nine of my girls start screaming, I know something is going on and I have been at home and able to run outside and scare the hawk away. There have been a few occasions where the hawk is on top of the tree or bush and my girls are screaming underneath of it. Hopefully the hawks will never get my chickens, but I know it can happen as it unfortunately happened to you. Try keeping lots of low cover for them. I try to have the bottoms of the trees and bushes only about a foot high off the ground. I try to make sure everything is within a few feet of each other and when they are out they just go from cover to cover. Hawks usually won’t leave themselves vulnerable by going onto the ground to get their prey and they sort of need flight room so if you have lots of obstacles up that makes it more difficult for them. There are a lot of hawks in my area and there is actually a nest a few doors away were some large hawks reside and raise their young.