Muscovy that is a good way of getting the flies under control.
And second option is fly traps. You can buy them at most farm stores you put water in and then this powder which is the fly attractant last about a month and usually has tons of flies when you dump and start over. These are for outside the coop inside best to just keep the poop cleaned out and maybe use something like barn limes or sweet PDZ in the bedding some use those sticky fly traps you hang up high so nothing can get caught on them But flies, I had a bantam chicken caught on one once so I don't use them inside any longer.
Hard to say on the voice they sure don't look sick though.

Hard to say on the voice they sure don't look sick though.