I leave my egg shells in the oven on a pie tin.  It has an old style pilot lite and dries everything.
Chickens get some of it, but they need 3-4 eggshells to make one egg, so they still need another source.
I also use my egg shells for gardening...add ground eggshells in the soil when planting  tomatoes, squashes, any blossom end rot suseptible plants.  THe calcium helps the plant fight off blossom end rot.
Also, spreading very small eggshell fragments under your brocoli/cauliflower type plants keeps supposedly makes those white butterflies think they'res already too many eggs around and they look else where.  Not sure if it really works, but this year I've had the least number of  those little green caterpillars I've ever had.
Sandra