Hello and welcome to BYC!
Sorry you are having troubles here. See those thin horizontal lines in the left feather in this picture? Those are caused by Quill Mites or Depluming Mites as they are also called. These are a microscopic mite that does not live on the skin of the bird but burrow up into the feather shaft, spending most of their lives up inside the feather consuming it as it grows. They especially love newly growing blood feathers and that is how these horizontal thin lines get there. These mites bear live young every 17 days inside the feather and then they come out of the shafts, briefly onto the skin and find another feather to go feed on and continue the cycle of breeding.
These Quill mites are usually uncommon to have to deal with, however they themselves are fairly common. Many birds have them, but a healthy immune system learns to deal with them so most people never have the need to treat for these mites. The very young, old or sick birds will have trouble controlling these mites.
Last year I introduced chicks to my flock that had terrible issue with these mites. At the time I didn't know what the issue was until it got so bad that I had to turn to a poultry vet. My birds looked hideous....bald butts and loins, bare wings, preen gland area was bald, necks, red skin, just awful. These mites irritate the bird and they will start with over preening their feathers, as you have here, stripping the shaft of all of it's plume, and then if the mites get too out of control as they did with my birds, they will chew them right off at the skin and sometimes even pluck the feather. These mites move fairly slowly from bird to bird and only through large infestations do they transfer through contact.
The only treatment of these mites is Ivermectin Pour on for Cattle. Comes in a yellow box. Dusts, and sprays will not do a thing for these mites since they spend very little time on the skin. You will need to order this stuff on line. What I did was depending on the size of your birds, standard large breed birds use 6 drops, 4 drops for bantam sized birds....You are going to use an eye dropper and drop these drops directly on the skin behind the neck. If they get on the feathers, it is useless. So make sure to hit the skin. If you accidentally give an extra drop or so, it won't kill them. Repeat this once a week for 4 or 5 weeks. This will make the skin toxic for quite sometime. This drug does not get up into the feathers, but after the young hatch and come down out of the feather and reach the skin, they will die. Since they hatch every 17 days you will need to keep this treatment up for 4 or 5 weeks to kill all the young and eventually the adults die inside the feathers. And you might take a month off and repeat this if they are still excessively preening, picking and chewing feathers.
Your birds will look ratty until they molt either this fall or next fall, depending on their age. I had 3 birds with these mites this past year. Each with varying degrees of looking ratty. One of them was just horrible looking. She molted last month and grew in a beautiful set of feathers...smooth sleek and very little signs of these mites.
Good luck and keep us posted! :-)