Get them some mixed grit. Its only about £4 and it's better to be safe than sorry. I put mine in a trough made out of drainpipe which they can access freely.They unfortunately don't get all they need from free ranging.Hens do lay the occasional shelless egg anyway so don't get too worried. I was once very amazed to see one of my hens crouch down in the outside pen,looking like it was about to have a poo like a dog :o ...out popped a shelless egg and lots of my other hens pounced on it and ate it. I have since not suffered mass egg eating and I would never kill any of them for doing it anyway. If you are really worried about egg eating try looking in the nest more often than usual and take the temptation away.