I have a mix of girls, mostly barnies. My black girls, who I believe are part barred rocks, have the dirtiest butts! They are healthy, happy, laying, doing everything as normal, but their butt feathers under the vent catch all of their poo. I have spent a good part of an hour soaking and cleaning their backsides, only to have it build up again after a couple of days. This is gross, what can I do?