Environmental conditions fluctuate across days, seasons and years. Animals adjust to this variation by displaying plasticity in behavior, physiology and morphology, which is often mediated by hormones. Hormone-mediated phenotypic plasticity is assumed to be adaptive, yet individual variation in hormonal responses to environmental variation can be substantial and attempts to relate it to fitness have proven challenging. I will review our work on characterizing patterns of individual variation in corticosterone (the avian glucocorticoid) concentrations in wild great tit populations and associating this variation with fitness traits like reproductive success. As hormonal traits themselves are plastic, with circulating concentrations changing within minutes, we have initiated a long-term field study in which we repeatedly monitor individuals along environmental gradients. Such reaction norm approaches hold promise for determining whether average corticosterone concentrations and their degree of plasticity in response to environmental variation are individual characteristics. Since recent studies have shown that glucocorticoid traits are heritable in avian taxa, future studies that include pedigree information will enable us to quantify selection pressures on and potential microevolutionary responses in hormonal traits of wild populations to changing environments. To understand possible costs of corticosterone responses we are studying the consequences of corticosterone responses for tissue function (oxidative stress and telomere dynamics). We hope that work of this kind will help us to predict the opportunities and limitations of wild populations for coping with the ongoing anthropogenic changes in their environment.