Unfortunately, calcium cannot be absorbed by the intestines, nor can its level be regulated, without the presence of Vitamin D3. The process of synthesis requires ultraviolet radiation of a specific wavelength. The most common source of this radiation is, of course, sunlight. Egg production can be increased by artificially creating longer periods of light, simulating natural seasonal light cycles. In most species, the breeding season has a linear relationship with the annual cycle in day length, with the breeding season coming to a close while daylight hours are still increasing.
Light also affects molting.
In some cases birds will molt continuously at a low level. When the feathers mature completely, the blood supply will dry up and the sheath will slowly flake away, exposing the new feather. Your bird's behavior is also affected by light. Intensity of light dictates the time at which birds go to roost, and when they emerge in the morning. Diurnal species depend on daytime light intensity to accomplish foraging and hunting.