HELP! Chickens are loosing bum feathers including roo. PICS

[Cruzin4us]

My chickens are losing feathers around their bums. At first I thought it was because I had too many roos with my hens but I got rid of all but 1 and I have 25 hens. Now I noticed my roo is experiencing the same problem. They free range when I'm home otherwise they are in. Critter Proof Fort Knox run. All are approx 6.5 months old. I watch them carefully and there are no feather pickers. I changed feed about 1 month ago from Purina Layer pellets to Kalmbach Natural Layer pellets that I puchased off Amazon. Could this be mites or fleas? They are getting a Sevin treatment tomorrow. I believe this isn't a molt. Thoughts?

 

[235chickens]

http://hencam.com/faq/bare-butts-feather-loss-and-feather-picking/

Hope this helps!!!!