I have 2 with the same bottom - bare and red. They look like baboons! Mine is down to one hen that has an unhealthy fascination with downy bottom fluff. She will peck and eat the feathers of any bird that is lower in the pecking order - that is how I worked out it was hen related and not an irritant. I tried antipeck spray but it didn't work - in fact it made both hens very acutely unwell. So I am just leaving it and hoping that eventually it will come back once the fascination goes. So far about 6 months like this. Doesn't seem to bother them however.