Birds need ultraviolet (UV) light emitted from the sun. It is UVB light, specifically, that induces Vitamin D production in birds, which helps them absorb calcium and assists other important bodily functions.
Full-spectrum lights emit light across the entire range of the possible lightbulb spectrum and some contain UVB in amounts beneficial to our avian friends. Because humankind has not, or cannot compete with nature, we are still unable to exactly mimic the light of the sun with manufactured materials. Therefore, no lightbulb can emit light that spans the full ultraviolet, visible, and infrared light spectrum the way that natural sunlight can. However, the avian full-spectrum lightbulbs with UVB that are available today better mimic the sun than ever before, thus are well suited for avian care.
So go ahead, bring in the sunshine with full-spectrum lighting.