I have Mockingbirds nest in my yard every year and when the babies leave the nest the parent birds will dive bomb the chickens to keep them away form the babies on the ground. After a few hits the chickens go out of their way not to get too close to the babies. I wish the Mockingbirds would stay around all year long, they keep the hawks away.
I came home from work yesterday to find a hawk inside my coop with hens on the nest and a broody with chicks. I didn't lose any hens, but one baby did get crushed in the panic. I guess it's scarecrow building time.