I would suggest you consult with an avian vet to get more info. It is common sense though, as humans, dogs, cats, etc. all need UV sunlight for Vit. D & the natural body process of this is much better than any Vit. D any living creature can obtain by a vitamin supplement. This is simply a biological & physiological fact. I have pasted a link to the Merck veterinary manual for more info. More about it is under the "calcium, phosphorus, vit. D imbalances" section of this page. There is also much more info. on various things here too. Sunlight is important for all living things metabolically. However, it is not necessary to have a UV bulb on them for hours & hours. My avian vet said about 20 min. outside daily or at a window that is not filtered by coated glass or a screen that blocks UV, is all that is really needed. I only turn on my UV bulbs in the house for 1 hr. for my parrots.
Learn about the veterinary topic of Nutritional Diseases of Pet Birds. Find specific details on this topic and related topics from the Merck Vet Manual.
www.merckvetmanual.com