I know others have said this but maybe they're bored. It looks like feather picking to me. They might start because of a need for protein but then they develop a habit and are bored so they pick. Part of my solution last year was to provide lots of bedding, straw mixed with shavings on their floor and throw grain into it. Gave them lots of exercise and the picking stopped. I've heard that straw bales can be infested with lice and so sometimes that is the culprit. Dusting works only if you do it on a regular basis, like every ten days... because the adults lay eggs (and I'm not even convinced of that because they all didn't lay their eggs the same day!) . The dust kills the adults but then the eggs hatch and it starts all over again. I provide DE mixed with shavings for my girls to dust themselves in and then on occasion I check them over and sometimes some will still have mites. I use Ivermectin pour-on for cattle. It kills the eggs too. It is produced for worming cattle, therefore, it is absorbed through their skin and it worms them at the same time that you're killing mites and/or lice and their eggs. As its made for cattle, you should be careful how much you get on their skin. I think there are different kinds of lice... some might be blood sucking and then you have the feather eating kind. I think these kind don't live on the bird but in the coop and they attack at night when the birds are on the roost and they chew their feathers. I had some that for the strangest reason only got on my birds that are black. They would chew the tails of my roosters off! You could clean your coop really well, getting the dust from the cracks and all and then spray the entire building with Sevin spray, being sure to spray the cracks where lice would hide.