What allows birds to lay eggs with hard shells?

The story behind the answer

Calcium is the key mineral that hardens bird eggshells. In the shell gland of the oviduct, birds deposit calcium carbonate (CaCO3) as calcite crystals, creating a tough, protective shell that’s still porous enough for gas exchange. Without enough calcium, birds lay thin or soft-shelled eggs.

Context check: keratin builds feathers and beaks, not shells; phosphorus mainly pairs with calcium in bones; magnesium is only a trace component in shells. Memory tip: eggshells, seashells, chalk, and limestone are all mostly calcium carbonate—think “calcium creates the casing.”