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Fluorescent Feathers Give Parrots Added Allure in Courtship, Study Finds

Bijal P. Trivedi
National Geographic Today
January 4, 2002
 
For most savvy individuals, fluorescent clothing is a fashion faux
pas tolerable only at '80s nostalgia parties. But for parrots,
fluorescent feathers are key to sex appeal.

That's the
conclusion of zoologist Katherine Arnold of the University of Glasgow
in the United Kingdom, who recently completed a study of fluorescence
in bird feathers.


Her findings, published in the January 4 issue of the journal Science, indicate that having a bright set of fluorescent feathers is an evolutionary advantage for parrots and not just a pretty aspect of their coloring.

Arnold first became interested in fluorescence in bird feathers after reading a note from a museum curator in a 1991 popular science journal. It said that dead parrots glow when placed under fluorescent light. (Fluorescent pigments "glow" because they absorb ultraviolet light, which is invisible to the human eye, and reemit light at longer wavelengths, which can be seen as bright whites, greens, and yellows.) Arnold decided to investigate.

She visited museums in Sydney and Melbourne, Australia, and looked at dead parrots. "I sat in a dark room with an ultraviolet light and looked at more than 600 dead parrots from more than 60 species and noted which feathers were fluorescent," she said.

Most of the feathers that Arnold found to be fluorescent are used in courtship displays. This led her to wonder whether fluorescent feathers had more visual punch and were more attractive to potential mates than dimmer feathers.

To determine whether fluorescent feathers were indeed more appealing, Arnold and her colleagues applied sunscreen lotion to the crown feathers of male and female budgerigars. Sunblock has the same effect on feathers as it does on human skin—it reduces UV absorption, and thus "dulls" the level of fluorescence.

Arnold found that fluorescent feathers were important to both male and female budgerigars in choosing a mate. The birds that were studied showed a significant preference for prospective mates with brightly glowing plummage compared with their sunscreen-slicked rivals.

Go With the Glow

Videos of the birds revealed that both male and females spent more time courting "glowing" companions. When the birds were exposed to companions of the same sex, the amount of fluorescence did not affect the social interactions.

To ensure that the birds were basing their choices of mate on looks and not on the smell of the sunscreen, Arnold used a nearly odorless sunscreen made directly from raw ingredients and without commercial additives such as perfumes.

From the results of the study, Arnold and her colleagues concluded that the fluorescence contributes to sexual signaling.

In some other species, such as peacocks, the female alone chooses her mate. For parrots, however, the match must be a mutual decision—clearly based in part on the amount of glow.

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.

Early-Morning Preening

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.

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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