my stepfather has some chickens that he has had for almost a year now and maybe few months ago his chickens feathers on the rear near vent and extended to whole bottoms are featherless. he has treated for mites and coop and has also have them antibiotics to no avail. does anyone have any ideas on what is the cause of this? i must also say that the rest of the chickens feathers looks fab from head to wings and only a few of the chickens out of 10 are affected... the rest look perfect. thanks for any info!