We have fly traps all around the chickens. It really helps, but there are some that refuse to go into the traps!

I got some of the fly parasites that
@BlueBaby got, I can see that they are hatching out, I hope they work. Of course when we got the big rain and hail last week, they fell down on the ground. Next time I will wire them up and not depend on the little paper hanger on the mesh bags they send you.
My birds will stand on a block of ice in a shallow roasting pan. I get the roasting and broiler pans from the thrift store for ~ $1.00 and freeze water in them then put it out for the chickens. I freeze 1/2 gallon plastic milk cartons and put it in their water bucket, too.