I would assume feather picking. You say your hens are fine which leads me to believe the hens are the ones causing the damage. I had the same thing happen in my Brahmas (when I had them).
Here is what I did:
I took a pair of nail clippers and barely clipped the end off of the top part of the beak. Literally, I barely made a dent, but it was enough that you could see the bottom beak jutting out barely ahead of the now "squared off" top beak. This keeps the beak from completely coming together and ruins the ability to cause damage as they are not able to get a good grip on the feathers to rip or pluck it out.
I did this to all hens. On the males, I sprayed Adams Flea and Tick spray to help with any mites and to make the feathers "taste" bad although I'm not certain that the birds can even taste it because they only have 24 taste buds to begin with. After that, I applied petroleum jelly to the entire leg. This gave it a bad texture for the birds that continued picking.
After this, the males grew their feathers back very quickly. I only had to do this once as once the birds realized they were not getting what they wanted (fresh feathers) they seemed to forget to do it.