My favorite hen, Big Red, who is in my profile pic, had a large patch of bare, red skin on her breast and belly. I bet it was depluming mites, but this is the first time I am hearing about what causes it. I think the depluming mites eventually kill the birds by sucking out all the nutrients from their bodies. Or maybe weakening the birds' immune systems. Either way, Big Red died several years ago. Hopefully I have learned this information on how to treat early enough to save the bird I have now who has very red patches by her butt, and many broken feathers. Thank you Two Crows!