Fruit for health: the effect of flavonoids on humoral immune response and food selection in a frugivorous bird

1. The intake of antioxidants confers health benefits to consumers by reducing oxidative stress and improving immune functions. Therefore, several life-history traits such as growth, immunity, senescence and the expression of sexually selected traits might be affected by dietary antioxidants. 2. Flavonoids are strong antioxidants in vitro, and are among the commonest found in fruits, which are a primary source of antioxidants for many animals. It is therefore likely that flavonoids play a beneficial role as dietary antioxidants, but their potential has been ignored in evolutionary ecology. 3. We investigated the ecological importance of flavonoids, using wild-caught blackcaps (Sylvia atricapilla), a frugivorous European songbird. 4. We verified the assumption that flavonoids can be absorbed and circulated by blackcaps. 5. In a food selection experiment, we showed that, when given a choice between food with and without fruit flavonoids, blackcaps actively select food with flavonoids. 6. We found a positive effect of flavonoids on humoral immune response. Birds supplemented for 4 weeks with a modest quantity of flavonoids were more likely to mount a humoral immune response after an immune-challenge, compared with control birds. 7. To conclude, our study demonstrates that birds select food with flavonoids and that these antioxidants increase consumer humoral immune response. Since these effects occurred at a moderate dose that is obtainable also by many omnivorous consumers, we suggest that flavonoids may play a beneficial role in ecological immunology for several wild species.