About baggies with water: On another site about backyard pests, I read that many people hang these around their doors and the thinking is that the reflecting/refracting light through the water repels the flies somehow. People on the pest-question sites said the baggies don't work well in open areas, like if you picnic in your yard, but did seem to help with flies sneaking into your house.
I recently ordered and sprinkled out the fly predators. I use stinky-bait traps in the yard. I have five hens in a backyard. They are in their pen/dog houses at night and free range in the day. I believe I'm doing a good job cleaning up after them in the pen. In the yard, however, they scatter droppings around in the wooded area, grass, etc. The flies are terrible. I'm in town and we've never had them like this. Other neighbors do, too. A hardware store I use said they've heard such things from neighborhoods all around town, leading me to think it's not just the chickens. No dogs nearby that are outdoors, no big garbage issue, etc. No compost piles.
I am considering an automatic pyrethrin mister but wondered about the safety with the chickens. Actually the more I think about it, the more I wonder if the chickens are worth it. A couple really are pets now...they climb onto our laps, and we like the eggs. But I can't eat in my yard anymore. I'm starting to have nightmares about flies!