I have several chickens that have large sores on their bottom just below their vents. I lost one of them last week. A second one is very lethargic and I don't think she will live through the night. I separated her from the others. I have a three others that act good but have the same large sores. Almost all their tail feathers are frizzy. I inspected for lice & mites; don't think I found any but quite honestly I am not entirely sure if I am checking correctly. Any ideas on what is going on? Anything I can do or check for? These are my first set of chickens and they are all less than a year old.