Tropical Asian birds have a penchant for red and black -- a proclivity that likely prompted jungle plants to sprout fruit in these colours, scientists said on Thursday.
It has long been suspected that an abundance of red and black fleshy fruits in nature was partly inspired by a pigment preference of the keen-eyed birds that eat them and spread the seeds.
A team in China decided to test the theory by determining once and for all whether birds do have colour favourites.
To eliminate possibly confounding influences of smell, shape or taste, the team manufactured artificial fruits from a mixture of apple, pear, banana, wheat and corn flour rolled into small balls and dyed black, red, yellow, green or blue using tasteless food colouring.
They then put the fruit balls in front of different species of bulbul and barbet birds native to tropical Xishuangbanna in southwest China.
The birds, both wild-caught and hand-reared, were left to choose for themselves what they wanted to eat.
All favoured red fruit, followed by black, the scientists found, and all avoided green ones, whose seeds in nature are generally dispersed by insects and other non-avian animals.
The results were similar in experiments with real fruit.
"The preferences were constant over time, supporting the hypothesis that bird colour preferences are a contributing factor driving fruit colour evolution in tropical Asia," the team wrote in the Nature journal Scientific Reports.