Hamburg, Germany  - Birds instinctively choose the fruit which is healthy and shun less health-giving food options, German researchers found.

Given a choice, birds flock to fruits with the highest levels of antioxidants known as flavonoids, which boost the immune system.

The German researchers offered a group of blackcaps, a common European summertime bird, a choice of two foods containing different amounts of flavonoids.

They found that the birds deliberately selected the food with added antioxidants.

Birds fed modest amounts of flavonoids over a period of four weeks developed stronger immune systems.

Dr Carlo Catoni, from the University of Freiburg, who led the study, said: "We fed the birds an amount of flavonoids that they would obtain by eating one to two blackberries, bilberries or elderberries a day.

"We used this modest intake of flavonoids because high quantities are only available during the limited time of maximum berry abundance.

"Our study shows for the first time that flavonoids are beneficial compounds that can boost the immune system in a living organism. We also found that wild birds actively select food containing flavonoids.

"Our results have important implications for the study of ecology and immunity in birds, and for the evolution of the relationship between plants and the birds and animals they rely on to disperse their seeds," Dr Catoni said.

Dark fruits such as blackberries, tend to contain higher levels of flavonoids. As they did in nature, the compounds made the foods used in the study darker.

Dr Catoni, whose research is reported in the British Ecological Society journal Functional Ecology, said: "We are confident that our results are due to a learned selection for flavonoid content and not due to innate selection of darker food per se."

Flavonoids, found in high concentrations in fruits and vegetables, are among the commonest antioxidants in nature.

Scientists believe antioxidants may protect against heart disease and cancer by mopping up destructive molecules called free radicals. (dpa)