I have the same issue with some of my ducks, I have 8 females and one male. Some of the females are fine but a couple have the same issue, bare wing feathers just look like skeletons for feathers. They get all flock for food, get to graze from 9 or 11 am till 8 pm. We have a pond and I ensure they have plenty of water at night. Only 2 or 3 of the females are laying now. Just had one female pass with no indication as to what happened. and she didn't have issues with her wings. I also have chickens and they are all fine. Will send pictures when I get some. Forgot to add I am near Tampa, FL