Red red fowl mite and they don't live on the bird, but in the wood of your coop, come out at night to feast on the flock. That's why they are refusing to roost. You need to treat the roost and the wood nearby with something~other than DE...won't do it~ so they cannot reach your birds. I've never had them but if I did, I'd get some pyrethrin spray and spray down the roosts and the walls they are connected to and keep that up every 2 wks or so. I'd also treat the birds with pyrethrin powder. This site says the red mites have become resistant to most chemical based treatments, but they didn't mention natural insecticides like pyrethrin. A chemical derivative of that is permethrin, easier to find but they may be resistant to it.
http://parasitipedia.net/index.php?option=com_content&view=article&id=2540&Itemid=2816
It's a good idea to get rid of any feed left out and only feed in single meals that are consumed at once.....red mites come from wild birds and mice that come to feed on the chicken feed, so eliminating the source of the problem will take out the vectors that are bringing these parasites into your coop. Any bird's nests on the buildings, tear 'em down and don't feed out in your run...it attracts wild birds. Don't let any feed spill or waste on the floor, it attracts mice and rats. Put out poison for those that might already exist there.
An ounce of prevention is worth a pound of cure, they say.