By smothering budgerigars in sunscreen, scientists have discovered that the birds use fluorescence to highlight their sexual attractiveness.
Neurobiologist Justin Marshall of the University of Queensland in Brisbane and his colleagues used the sunscreen to block the fluorescence of yellow crown and cheek feathers used by the budgies in courtship displays.
The team found that budgies preferred the company of control birds, whose crown and cheeks had been smeared with petroleum jelly, which does not block UV light. The preference only existed for birds of the opposite sex, suggesting the fluorescence highlights mating potential.
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Fluorescent chemicals are relatively common in nature, particularly among other members of the parrot family. But this is the first research to show that these pigments can be used in sexual signalling, says Marshall.
Walter Boles, an ornithologist at the Australian Museum in Sydney, says showing that fluorescence has a function is rare. 鈥淭he butter in your fridge probably fluoresces if you shine a UV light on it, but it isn鈥檛 saying 鈥榚at me鈥�,鈥� he says. 鈥淏ut here the birds are using it in mate selection.鈥�
Double cones
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 colour.
A budgie鈥檚 visual system may be specially adapted to detect fluorescence, says Marshall. For instance, special receptors called double cones in the birds鈥� retinas are particularly sensitive to the wavelength of the fluorescent yellow.
This suggests that the budgies鈥� 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 鈥� which is unique to parrots 鈥� is biochemically costly, meaning it may be a more honest indicator of fitness than ordinary colouring.