First, if you have laying hens, make sure you're feeding them a layer feed. This will automatically contain higher levels of calcium. But that alone is usually not enough.
It's best to also supplement their diet with additional calcium. This can be done in more than one way. There is commercial calcium for chickens at any feed store. This is generally coursely ground lime or oyster shell and comes in small bags that are perfect for the urban chicken owner.
Placing small containers of calcium (and also one of grit) in the chicken run allows the chickens to eat as much calcium as they want, when they need it. Their bodies will tell them how much.
Another option is to use the hen's egg shells. To do this, it's important to bake the shells first. This will kill any lurking bacteria in the shells, change the flavor of the shells (so that your hens won't get any ideas about pecking their own eggs after they've tasted the shells), and also softens them a bit.
Baking shells are easy. Just place them on a cookie sheet and bake for around 30 minutes with an oven set to 225 degrees. When they're done, place them in a large bag and crush with a rolling pin. Now, they're ready to serve.
Some chicken owners say never to feed shells to your chickens for fear of them eating their own eggs. I've never had that problem though. The crushed shells are so popular with my hens, I mix them with oyster shell to make them last longer. And still, even with their popularity, I haven't had to deal with them eating their eggs before I can get to them.
Egg shells are a great source of calcium and I love the fact that they're recycled. Since becoming a chicken farmer, I understand the preciousness of each egg that's laid, so it's wonderful to be able to use the whole egg, not just the insides.
(Here's more ideas on how to use those shells if this idea doesn't appeal to you.)
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