Bald undercarriage

One of my rangers is totally bald underneath. If you tip her up she looks like an oven ready chicken!! Weirdly though there are no signs of any feathers in the nest box, coop or covered run. We have checked and re-checked for any sign of red mite in the coop but can find nothing. Is it a bit late/early in the year for the moult? She is about 16 months old and has been laying approx 5 eggs a week through the winter, although the shells are rougher and not as strong as usual. They get Heygates layers pellets, grit/shell and water plus a handful of corn each before bed. They also free range for part of the day so they get access to grass. Advice please?

Thanks for the advice. Re the corn, I thought you were supposed to give them some corn last thing to sustain them during the night. Have I been give incorrect advice? 

I had a speckled hen that was bald from crop to vent nearly all her life, no infestation or dietary problem involved.

I put it down to the fact that she was broody so often that she didn`t think it was worth getting dressed !

Half my girls have bald undercarriages! We have a couple of bullies and we do find a few big feathers lying around - not enough though!

We did see our Grey Marans Gertie eating the small fluffy feathers as she pecked them out?!