JubileeFarmer
Songster
I have hatched two sets of chicks under broody hens and both times when I collected the empty egg shells one shell was missing. I searched the nest box and the floor below the nest and couldn't find the missing shells so I am assuming that the hens ate them. It probably fulfills some nutritional deficiency. Perhaps your hen just needs the egg shell in her diet so badly that she isn't waiting for the chicks to hatch.