I have a large plastic bin (you know the kind that go under your bed with wheels and lid) full of play sand for my girls to dust bathe in. I take the lid off the sandbox and the first thing they all do is come running and eat the sand? I have only seen one chicken actually doing the dust bath in there. They have oyster shells next to their food. They range all over the yard, so they get lots of dirt, snail shells, muck from the ground. Is this bad that they all want to eat the sand? Is there something that I should be doing to discourage them? Or is it okay since grit is a part of their diet?