Calcium requirement for chickens changes as they age. They are also affected by local weather conditions and whether the chickens are laying eggs or not. Hens laying eggs need more calcium in comparison to those chickens that are bred for their meat. If the hens don’t get enough calcium, they might wind up laying eggs with very thin shells or even without shells.
Their shells will also be prone to breaking extremely fast.
Older hens need more calcium than the younger hens and the requirement increases in hot weather, due to older chickens not eating enough food. Ground oyster shells and ground limestone are good sources of calcium, just make sure you don’t feed the harmful dolomite limestone to your chickens.