I've got a new Bantam breeding pair in an aviary on our porch. The ground of the aviary is covered with yellow construction site sand that is relatively fine. Is that good enough for grit or too fine? Should I put some natural earth with pebbles in as well? I don't think there is anywhere to buy professional grit here. I don't want to give them crushed egg shells because the hen isn't laying now and their feed has too much calcium already.