How Does A Chicken Make The Shell Of Its Eggs?

The blood of a laying chicken at any one time contains only 25 milligrams of calcium (non-layers and roosters have only about one-third as much), yet while producing an eggshell the chicken uses 125 milligrams per hour. She clearly needs to get extra calcium? The original source, of course, is the food she eats, the chicken extracting the calcium from her intestines. But still she is unable to absorb calcium from this source as quickly as she needs it.
She draws it from calcium reserves found in the 'medullary bones' located within the cavities of most of her regular bones. These secondary bones are not found in male chickens or in chickens too young or too old to lay eggs. This system is so efficient that a hen can mobilize as much as 10 percent of the total calcium in her bones in one day if her diet is very low in calcium. However, if her diet continues low she cannot keep this up and so first compensates by laying eggs with thinner shells. If a serious shortage of calcium persists, the chicken quits laying altogether rather than laying eggs without shells.

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