This is probably a dumb question. I have a broody Marans hen who has a good sized clutch of eggs about to hatch. Yesterday I moved her and her eggs into a brooder pen because another hen kept raiding her nesting box and actually killed a chick. However I put a nest box in there that was too small for her so I switched it out tonight for a bigger one. In the process, I saw an egg that looked like it was missing a quarter of its shell but still had the membrane intact. As I was carefully moving eggs from one nest box to the other (while holding the hen under one arm), I realized that the "broken" egg was chirping and attempting to zip. I quickly moved the new nest box into place and put the hen back in the pen. She promptly settled back onto the eggs but didn't pull the "broken" one completely under her. The way she is positioned it looks like perhaps she is carefully chipping away at the shell to assist. I've never witnessed a hen do that before....
