Sore tail

[erin29]

My isa brown has lost her tail feathers, she has the "vain" part of the feather but nothing else, in front of her tail there is a thin long lump, she doesn't seem bothered by it at all.
I have recently treated her (and my other chicken) and the bedding for mites. I've been giving them cat food for the added protein they also get treats of probiotic yoghurt, she seems otherwise fine and there doesn't appear to be any pecking from the other bird.
Has anyone else had this problem and found a solution?

 

[LaynaDon95]

Do you know how she lost her feathers? I think what you are describing as a "thin long lump" is her oil gland. Does it look like this? (Not the best pic, but it's what I could find.) Check her for mites, but she could just be molting. The extra protein will help her regrow her feathers.