I have a fake owl in my garden: a hawk alit onto the fence not 3 feet away from it, checking it out. Try to find one that has a bobble head, at least. It needs to move or animals realize something is 'off'...
I leave my dogs out when the chickens are out. Before we adopted him, the farm dog caught a flying hawk in their barn. I have been training him to respond to hawk cries: when I hear one, I say, "bad bird!" and run outside. He now alerts to their cries and goes out without prompting (from the porch). I focused on when they were pretty close and the cries were louder, and he does, too. He does not alert when one is down the road: just in neighboring yards.
I think the hawks would be more likely to go after a full grown chicken if it thought it would be safe to eat right where it killed one, since it could not carry one off. The dogs take that option away. I have no proof of this, except one incident a month ago when a hawk swooped down towards the chicken (the hawk was only 10 feet off the ground) then it just flew away when the dogs moved.
If I did not have the dogs, I would not let my chickens roam unless I was with them.