FLUORESCENCE IN BUDGIES
The fluorescence in the feathers occurs when special pigments absorb short wavelength light such as UV and then re-emit the photons at a wavelength in the optical spectrum, creating a super bright color.
A budgie’s visual system may be specially adapted to detect fluorescence, says Marshall. For instance, special receptors called double cones in the birds’ retinas are particularly sensitive to the wavelength of the fluorescent yellow.
This suggests that the budgies’ fluorescent feathers and visual system are parts of a sexual communication system that has been honed by eons of evolution.
The team speculates that the budgies are attracted by fluorescence because producing the budgie pigment – which is unique to parrots – is biochemically costly, meaning it may be a more honest indicator of fitness than ordinary coloring. (source)
In the wild, said Arnold, budgerigars perform courtship displays in the early morning, when sunlight contains the highest proportion of UV light—the time when their feathers would glow the most brightly.
"The birds puff up their feathers on their crown and cheeks and bob their heads up and down and from side to side to catch the maximum amount of sunlight and emit the brightest glow," she said.
Whether there is a link between the brightness of the feathers and some other traits is not known. Arnold observed, however, that older and more "worn" feathers emitted less fluorescence than younger ones, which may serve as an indicator of a bird's health.(source)