UVB is a necessary part of the process in which vitamin D3 is produced in the skin of humans and animals, and also what causes sunburn in humans. The vitamin D3 that is produced is responsible for regulating calcium metabolism. A lack of UVB and vitamin D3 can ultimately result in metabolic bone disease in reptiles, and rickets In humans. Nearly all animals that are active during the day are exposed to UVB. This includes birds living in heavily shaded tropical forests, and birds in open areas with direct sunlight exposure. Even though birds are exposed to UVB in their natural habitat, UVB has been traditionally ignored for pet birds. We know from science and experience that UVB is beneficial, if not absolutely necessary for humans and most reptiles. We now know that birds also benefit from UVB. There was a good article in the journal .Exotic DVM. Recently on .The Effect of UV-B Lighting Supplementation in African Grey Parrots.. This study determined that exposure to UVB with African Grey Parrots resulted in higher blood levels of vitamin D3 as compared to Parrots without exposure to UVB lighting, with and without vitamin D3 In their diet. The author found that the parrots were able to meet their vitamin D3 requirements through UVB. Alone, rather than through the presence of Vitamin D3 in their diet. The author states in his article that .it would appear prudent to supply both an adequate diet and UV-B lighting to grey parrots in order to prevent clinical manifestations of hypocalcemia in this species. And .the author now routinely recommends provision of UV-B Lighting to African species..
An article explaining one method that birds use to synthesize UVB that involves the preen (Uropygial) gland.
In this process, Vitamin D3 precursors are secreted by the preen gland. These precursors are spread over the birds feathers when the bird preens. These precursors and then exposed to UVB and converted to active vitamin D3, and then ingested upon future preening. As some birds lack ( any Amazona Species) A preen gland, birds are also able to produce vitamin D3 through the exposure of the skin on their legs and near their beak to UVB.
Vitamin D is a fat soluble vitamin, too much dietary vitamin D can result in an overdose. This is actually how many rodent poisons work. With UVB induced photosynthesis of vitamin D3, the risk of overdose is potentially reduced since his has been discovered to be a biologically regulated process. UVB is necessary for the physiological health of pet birds.