My hen is a 1 year old Wyandotte bantam cross friesian fowl. She lives 100% free range and sleeps in an old oak tree at night and forages in the day. She hatched chicks last year in a bush and they all survived so she lives with lots of other chickens and guineafowl in the tree. However after her moult at the end of summer I noticed her tail was held permanently pointing down, touching the floor. Other than that she seemed fine but I caught her and she had some scabs on her tail which I put down to rain scold as we had recently had a storm. I left her out but that was back in October, she didn’t improve but didn’t get worse however now she has taken a turn so I brought her into a run but it hasn’t helped, she’s very quite and spends most of her time lying in the coop, she struggles to fly up onto a low fence where she was once able to fly into the top of the tree from the floor and she’s now very easy to catch. Her tail bone is no longer distinguished from her body it’s just one mass of scab, blood and puss and her feathers are all falling out, it is spreading along her back and around her vent. Any ideas as to what it could be, no other birds are exhibiting this issue.