It helps to give the chickens a lot of cover. Heaps of low shrubs, but also low platforms they can rush under. Plenty of vertical obstacles like bamboo bean tripods can also help by interfering with ambush and lines of flight. It's easy to train a vine to grow up the tripod, and if it's large enough it can give chickens that extra edge (as well as a temporary hiding spot).
 
If they are penned they need netting over the top. However even that can fail -- a goshawk once caught itself in my pen, as it had slipped in through a 10 inch hole and couldn't find the way out.
 
We seem to have every size aerial predator possible here -- tiny sparrowhawks (not much danger but they do alarm the birds), goshawks and kites, and then some fairly insistent sea eagles, which nest about 300m away. Every year a junior sea eagle comes to harass the chickens, which I suppose is its way of learning what's what. I haven't lost a chicken to one yet, but that's because I pen the birds when the sea eagles are visiting, and their first few fly-overs are obviously scouting ones.
 
I find that there's a stable point reached when the chickens have learned to listen to each other and watch out above (roosters are especially vigilant) and there's a decent amount of nearby cover to break up the airspace as much as possible. A cluttered yard is better than a cleared one for free ranging (though then you're providing cover for all the ground predators, drat).
 
Basically all life is a trade-off... But despite having many aerial predators I rarely lose birds.
 
Best wishes,
Erica