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.