I have 8 laying hens and two older ones about 3 years old. The 8 were all chicks raised up. I see that 8 of the 10 have no feathers on their butts. One older one and one younger one still have their feathers. I watched them for a few hours the other day and could not find anyone pecking the feathers. I checked for mites and found none. What could be causing the feather loss on their butts. Feather loss is about a baseball size around their vent. I am feeding a 16% protein layer crumble, and scratch grain. No other supplements.