I would suggest either home made or purchased fly traps to help cut the population. Also look closely at them because not all flies are fliesI was at a race yesterday when my husband texted to say the neighbors scared a coyote off of our shared wall. The woman saw the coyote jump up to the corner and walk along the wall--she has a vested interest in keeping coyotes out of her yard as she has a tiny yappy dog. She shouted and scared it away, but we now know ONE way the coyote is getting into our yard. I'm putting a 2x4 spiked with nails on the corner and along the wall for a lovely surprise. Anyway, chickens are fine. I went over the layer coop and meatie pen again and looked for any possible weakness.
There is such a huge difference now between the cx and the cochin peanuts. My husband asked if the peanuts were sickly! There are way too many birds for the pen, so I'm going to butcher some CX this weekend. They'll be 5 weeks. I'm not sure how much they'll weigh. Bigger than market cornish game hens, for sure, but smaller than 5lbs.
All the birds had a great weekend. Hours out in the garden, pecking grass, dustbathing, running around, getting grubs from the compost, tons of fresh veggies. I did dig a pit as an official dust bathing pit, filled with sand, some garden soil, and DE, but as of yet, they haven't used it.
Oh my gerd, the FLIES HAVE BEEN HORRENDOUS. They are all over my little patch of lawn, and around the coops....though not IN the coops. The buzz is constant. I keep the area clean, so I dunno what is going on. I sprinkled DE everywhere but need a solution that actually works. I've about given up on any supposed benefit of DE. It is in no sense the miracle that many of BYC claim it to be.

All flies, bees, wasps, butterflies, ect are drawn to any damp spot when it's so dry, including fresh manure, they are looking for moisture. Also with the weather being so warm you will notice a sudden large population because they are hatching out from where they pupated last fall. If you can get the population down now you will have less later. I have found the permethrin goat shed spray at TSC will kill them; had a huge swarm of house flies congregating at one spot in the yard, went out and sprayed them when it was cool early in the morning and got about 90% of them gone. This spray is recommended for use in poultry houses so won't hurt your chickens.