I feed mine egg shells, sometimes I crush them sometimes I dont. I've never purchased oyster shells and only have had 1 shelless egg. It came from one of my toung pullet silkies. It think it was her first egg. Also , are the girls young or old?Sometimes young pullets and old hens lay shellles eggs when their system is ajusting to start laying or stop laying with old age. Sorry , not much help its hard to tell why chickens do weird things.