Based on previous studies of the retina in the birds' eyes, which have been done to determine sensitivity to various wavelengths of light, Arnold and her colleagues calculated that the fluorescence increased the brightness of feathers by 14 percent.
In the wild, said Arnold, budgerigars perform courtship displays in the early morning, when sunlight contains the highest proportion of UV lightthe 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.
An Italian biochemist collaborating with Arnold isolated the pigment molecules responsible for the fluorescence. "The structure of the pigment is very unique and very complex and requires a lot of energy to make," said Arnold.
Arnold noted that, curiously, many species of parrots that live in rain forests have not evolved fluorescent pigmentation. This is logical because there is generally very little light in a rain forest.
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