My two cents?
Feathers are made almost entirely of a tough, fibrous protein called beta-keratin, the same material found in beaks and reptile scales. While calcium is not for feather development in pullets, they need it to produce feathers as the body works in sequence.
While feathers don’t require calcium, a bird’s skeletal, muscular, and nervous systems do, and this is especially true for growing chicks and pullets, or young hens preparing for lay. A calcium-deficient diet can lead to rickets, skeletal deformities, weakness, poor development and even the whirlies, and in laying birds, soft-shelled or shell-less eggs, egg binding, and even death.
Yet, in some small-scale or budget-conscious chicken-keeping communities and food comapnies who dont want to dish out the dough...there's an unfounded fear surrounding calcium-rich feeds (note the studies on this subject, are subject). Some avoid offering proper calcium out of worry it’s expensive or unnecessary. This misunderstanding is not only dangerous—it’s inhumane.
You can’t have a diet void of calcium—I would know—I'm a scientist currently studying this, and I’m actively working to debunk the myth that calcium is a costly or optional part of a chicken’s diet. In truth, properly balanced feed—whether layer, grower, or chick formula—is carefully calculated to provide what birds need without
Depending on your soil- it may contain adequate calcium- and there is a tiny bit in insects exoskeleton. But you need to let them out often.