The same thing has happened to my EE, Susie. It's been happening to her for about a month. I've lost count how many times I've inspected her for lice/mites with a magnifying glass and seen nothing. I've dusted all my chickens twice within the time requirements as well as finishing a worming cycle. The feathers are broke in half and there isnt a rooster involved. I'm certain it's due to Susie being rather low on the pecking order and gets pecked on quite abit when I give them table scraps. I think it also happens when they go to roost at night, positioning themselves for the best roost, I hear squawking and picking going on in their house. There's plenty of room too. The heat has been unbearable this summer and that could be stressing her as well. I think Susie is starting to moult also, she's 1.5 years old, I've observed her losing a few feathers from her underside. As a matter of fact, currently half my hens are in moult. beachchickie, I hope this helps some, just some reasons and no solutions...I think it's just going to be a wait and see kinda deal unless someone else comes up with something.
