My hen has lost all the feathers around her vent. She is one of four hens, no roosters. THe others are not experiencing this problem. CAn not find lice or mites, have thoroughly cleaned the coop and am using wood ash in the ranging area. Her skin is very reddened and looks sore. Any suggestions what might be causing this. Other than that she is healthy, laying well, eating and drinking and socializing with the others as normal.