One of the things about sustainability is that you reduce waste and reduce inputs and re purpose waste as a positive element in the system of whatever it is you are doing.
One of the things we do with our chickens is save the eggshells and when baking in the oven we add a tray of used eggshells to bake out any moisture and dry out the shells. Duck or chicken it makes no difference.
So after we bake the eggshells in the oven....... this is what they look like. The next stage is to put them in the mortar and crush them and fit about 8 egg halves into the mortar........
Then with the pestle we crush them to make them palatable for chickens and ready to add to their food.