What sort of cover do they have other than trees and how wary are the chickens of the aerial predators? Maybe a number of low (meaning chicken low, not people low) structures, lean-tos, A frames, etc would give them somewhere to escape in a hurry. My chickens like to hang out under the lilac bushes and the pool deck which has lattice all around except where it joins another deck which is attached to the house - that is their "door" and I really doubt an aerial predator would try to get in. The lilac bushes are useful only when in leaf of course and around here that means mid May to November.
Is this normal raptor migration season for your area and are you on a known migration path? If so, it might be worth noting the spring and fall migration times on your calendar just to remind you it is time to keep the birds in.
At the very least, though terribly frustrating as it has been, it sounds like you have added protection for most every predator you get one at a time and next year should be a lot better!
Best of luck.