Normally the rabbits get plenty of calcium in their diet without supplements... but it is helpful to give kindling does a little calcium boost to avoid problems. Alfalfa hay has a lot of calcium.
You may find this link interesting:
http://shady-acres.com/susan/Calcium-Phosphorus.shtml
You may find this link interesting:
http://shady-acres.com/susan/Calcium-Phosphorus.shtml