Just read the Countryside article, "Portable Fly Control: (and its environmentally friendly!)," and it was great! (Vol 94, No 3) It is about the Epps Biting Fly Trap, invented by cattleman Alan Epps. Flies are attracted to the sillouette of animals, so these large, part dark and part transparent sheets attract flies. They fly into the clear plastic which mimics the underbelly of an animal, and bounce off of it into a tray of water. You put 8 drops of soap in the water of any kind (LOC is drinkable by livestock), which breaks the surface tension of the water so the flies sink rather than float. It will catch about a pound of flies a day (gross!), and clear an area 2-4 acres of them. You replace the clear plastic sheets for $8 every few years, but that is much better than the chemical approach, which many insects eventually develop resistance to anyway. Even the lightweight, portable aluminum version can withstand 90 mph winds (held down by 2 sandbags), so the trap can be moved every time cattle switch pasture. The portable trap goes for $299, and can be gotten from www.horselineproducts.com
Also read, "Fly Control without Chemicals" in Backhome, no 113, which also shows a fly trap. They additionally talk about prevention with sanitation, and utilizing predator wasps and dung beetles. The wasps can reduce fly larvae by 90%, and dung beetles in around manure can bury or destroy 95% of horn fly eggs and larvae and 90% of other cattle parasites.
A question non completly on-topic, but non-completly off-topic either
Fly trap are well known (for example the bottle cut in two with the upper part flipped) but i'm trying to figure what coulld be a fly trapper that can be at the same time chicken feeder. But i can't figure how the chicken could catch the flies and flies couldn't escape.
Hmm. I bet you could set up something similar to what they do with horses (Epps Biting Fly Trap), where the flies fall into water and drown. If it were shallow water, maybe the chickens could eat from it?