I had the same problem earlier this summer. I was providing oyster shell free choice too, so that was not my problem. I was advised (by BYCers) to put golf balls, wooden or ceramic eggs into the boxes - a lot of them. That's what I did (golf balls - 3 per nest box). My problem was solved immediately!! I left them in there for a week & tried to remove them. The eggs got attacked again.
I put back the golf balls & left them there for weeks. I was finally able to remove them without any egg eating.
Hang in there!


Hang in there!