When my hens were eating the eggs, there was very little evidence aside for increase in number of eggs and sticky hay in the nest box. They ate shell and all. Sounds like yours are poking holes as @oldhenlikesdogs said. I would try what the other posters have recommended and if at all possible, I'd try to cover the nest boxes. If they only have room to climb in and out, they are less likely to trample over top of the boxes. Keep us posted