I have heard that crow decoys and crow sounds will attract more crows
they will help drive the hawks away
there are some good crow decoys at cabellas site
hanging CDs, poulty netting, giving your chickens lots of places to duck under
and knowing that if you free range you are going to lose chickens are all part of it
keeping them in a large enclosure with a wire covering or roof is the only sure protection from hawks
no matter how many you kill, more will follow
good luck with your flock