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BIRDS may choose ripe fruits because of the ultraviolet light they reflect,
rather than being enticed by a particular colour, according to a new theory.

It might explain why fruits of one colour aren鈥檛 necessarily better than
others at having their seeds dispersed by birds. In temperate regions, birds eat
mostly black or red fruits. But tropical birds disperse the seeds of fruits with
a riot of different colours.

Douglas Altshuler of Purdue University in West Lafayette, Indiana, studied
fruits from 57 plant species on Barro Colorado Island in Panama. He found that
fruits that reflect UV light were always conspicuous to birds against green
leaves, whether in sunny or shady habitats. The more a fruit reflected UV light
the better birds scattered the seeds.

Crucially, unripe fruit did not reflect UV light. 鈥淚n fruits with UV colour,
it is a signal that the fruits are ripe and ready to be eaten,鈥� he says. 鈥淭his
is the first study to link a particular colour, in this case UV, with specific
诲颈蝉辫别谤蝉别谤蝉.鈥�

To check his idea further, Altshuler and his team placed UV-absorbing filters
high above a species of a low-growing plant called Psychotria emetica.
The plant鈥檚 fruits strongly reflect UV light and are mainly eaten by a
ground-foraging dove and migratory thrushes. They found that the birds removed
nearly all the fruits from plants that were allowed to bask in UV light, but
removed fewer than two-fifths of the fruits from plants that had no UV reaching
them.

Researchers have known for 30 years that birds can see UV light, and
Altshuler鈥檚 conclusions makes a lot of sense, says Charles Janson, an
evolutionary ecologist at the State University of New York at Stony Brook. 鈥淚n
addition to the difference in colours that we perceive among bird fruits in the
tropics, the birds may be perceiving yet another whole dimension of colour that
we are missing,鈥� he says.