Hawks have far better eyesight than chickens, and if caught out in the open or if protecting chicks, a hen will squat and hover, making her the perfect meal. Chickens have SOME instinct left, amount depending on the breed and how they were raised, but the ARE domestic birds, and low on the food-chain, at that. Old English Game hens make great foster mothers, and have more instinctive behaviors retained than most egg-laying breeds, and they focus on teaching "their" chicks. Even silkie chicks become more savvy with an OEG "mom", IF you trim the feathers around their eyes....
Free-ranging is a quality-of-life issue. Moveable "chicken tractors" can help.
We let ours out when we are home, and so are the dogs. If I hear a hawk "ki-iii'ing", they go back to our covered pen until later. Find out what hawks are in your area, and go to website (allaboutbirds) to learn their calls; you can hear them from a good distance.
Hawks will set up daily "watch" over a known food supply--bird feeders, or your chickens. If you count on "unknown objects" to dissuade them, be sure to move them at least once daily--hawks aren't stupid.
If you see one perched and observing your birds, your "head-count" is about to go down. Penning the chickens until the hawk gives up (usually about two weeks) will decrease your losses.
BTW; you can't kill them; they are protected. Hefty fines and bad karma.
