Ingredients
equal parts of,
Plaster of Paris (available at craft shops or Pharmacy)
Calcium Carbonate
Garden lime
Method
Mix these together with enough water to make mixture similar to pouring custard (don't have it too wet). Pour into containers/moulds and add a piece of wire shaped like a U. Have about 2" protruding from the top, this is used to attach the block to something in the aviary. For moulds you can use :- Plastic cups, muffin tins, empty fast food containers. Allow to dry before removing from the mould.
You can also add your own vitamin or minerals or maybe a little grit to this mix. Note : the calcium MUST be "calcium carbonate".