If you can get some Stall Dri or some Sweet PDZ from your local farm place or
TSC, I'd do that first off. It dries up poo very quickly so the flies don't have something to lay thier egs on.
Sprinkle some of that around the yard, and especially on the poo that is fresh, clean the coop top to bottom and then sprinkle some of that around on the ground, then if you have it, add some DE to the nest boxes and roosts, etc to make sure that the nest's are free of flies, mites, lice, etc.
Cleaning a coop is very important, you can check around the site to see what type of coop you have and how you should clean it. With the hen's heads being bloody, can you get some Blue-Cote to help that so they aren't picking at each other?
That must be a bad infestation of flies!!
One thing that I picked up from here and other messageboards is to take a water filled ziplock bag and hang it over the coop door, or wherever the flies are bothering the hens, and just let it sit there.
Flies cannot STAND it! I don't know why, unless it's thier vision that messes with them, but they HATE seeing that water.
The other remedy that is natural is the get a few 2 liter bottles, cut the top 1/3 off of them, rinse, add 2 cups water, 1 cup vinegar, 1 cup sugar, and if you have it, a tablespoon or two of honey. Put that mix into the bottom half, then turn the 1/3 rd of the top updside down, and either let it hang or place near a spot where the flies like to gather.
Kind of like a yellow jacket trap, just bigger.
Hope that helps!