The phosphors which produce ultraviolet light are different from the mixture employed to produce visible light. They degrade at a much quicker rate. This is roughly 250 days of use, after which the bird would have to be sitting directly on top of the fluorescent tube itself to receive benefit, if any. Another problem concerns avian visual and endocrine disturbances from the visible and UVA output of the lamp at close range. Light plays an even more important role for birds than for humans in metabolic control.
Avian forms gather information about light quality from the retina.
This information is then relayed back to the pituitary gland. Taken together, these two pathways set the pace for all subsequent endocrine regulated metabolic processes. Concentrations of UVB in fluorescent tubes can be increased, but due to the amount of barium based phosphors involved, these devices are no longer lamps which can produce a suitable visible spectrum. The effective life span of these devices is also short, as noted by the previous discussion. This problem may be compounded if your bird's diet is deficient in Vitamin A, and this protection is weakened.