Fly predators they r little wasp with no stingers and you just sprinkle them around your chicken coop on the out side and were u keep there poop and they go in and eat the fly larvae before they hatch into flies u do this every month from spring to fall and it will if not cure it will cut them back I do it and it works well.
Hi we had a lot of flies around the duck pen. I found this idea on Pinterest. (And 1st saw it at our local ice cream place lol)
Take several small clear plastic bag ,fill with water and a couple Shiny pennies. Tie with a string and hang .
I put 3 bags around the corners of the pen and the flies are gone.