Growing animals face allocation problems whenever receiving suboptimal amounts of resources in very stochastic natural environments, possibly through a trade-off between growth and somatic maintenance. However, the extent to which such a trade-off exists has remained an open question. We used an insulin-like growth factor 1 (IGF-1) injection treatment in free-living pied flycatcher (Ficedula hypoleuca) nestlings to see how IGF-1 levels mediate the development of an antioxidant phenotype via glutathione peroxidase (GPx). Our study showed that IGF-1 levels underlie variation in GPx activity and provides important information for understanding the mechanisms behind the growth variation of passerines.