I have one Australorp hen that my rooster favors. (He favors all the Australorps, but this one in particular.) She has been missing the feathers on the back of her neck and base of tail for months from all the attention. Tonight while digging around for worms with them, I saw that her belly/vent area is now balding. I picked her up and she has no mites, lice, or wounds. The skin looks perfectly normal and healthy. I felt for any growths or abscesses under the skin - nada. Her vent looked normal too with no discharge. What else should I be looking for? I spray Vetrycin on the reddish skin at the tail. The skin isn't broken, but slightly irritated.