My flock doesn't have access to any grit (their outside run is bare earth with mulched leaves and they only get to free-range for about 15 minutes every other day, under supervision) - anyways the ground is starting to freeze here... tons of snow soon. Can I give them dry sand in a bowl, would they peck at that as grit? Or is that too fine? What could I buy at the co-op? They're not helpful there... Thanks!