One of my hens, Sweetie, has had this for a couple of months or just under, I think. She did have a fairly large bum when I brought her, but only recently has this developed. I didn't really notice it becoming bald, I just happened to realise one day that her bum didn't have many feathers and it was quite red. I have been washing it every other day, to get rid of the faeces etc, but it doesn't seem to be going away.. I assume she has runny faeces, as it goes all down her bum, but that is just a thought. None of my other hens (3 others) have this. She seems well and is lively and eats a lot (and she is rather heavy compared to the others!), I'm just a bit worried if there is anything actually wrong. I will take her to the vets if no one knows what the problem is, and I can always buy some medication if it comes to it. She is quite tame, plus I have noticed that some of the hens peck each other's feathers to eat them ( not in a nasty way for the pecking order) - could this be a possibility for this? I look after their house for red mite quite often too. A few times I have found her sitting in the nest box at night instead of on the ladder thing, but I don't know if that has anything to do with this - it's not a usual thing she does. Can anyone help please?