I have a couple of chickens starting to lose feathers at the base of their tails on their back by their oil gland. One has lost an area of about 3" x 1", and the other 2 are just starting to lose. They are around 11 months old. I feed them the same organic food and they freerange in my yard. They get daily treats from scratch to fresh produce. I'm not sure if this is molting or if they are having something. Any suggestions would be helpful, thanks!!