Some birds require more calcium than others which is why it's good to always have fine oyster shell grit on offer in a separate bowl. The ones that need it can top themselves up. Sufficient calcium also stops them getting egg bound. It's the same stuff you'd feed to a pet budgie or similar. The pieces in the one sold for chickens are generally too big. You can use egg shells but it's not as potent a source of calcium so they will need to eat more of it to get the same benefit. Hopefully some extra calcium should help and your girl stops laying you rubber eggs.
