To estimate the past and future impact of urbanisation on birds, it is important to understand the mechanistic underpinnings of a physiological response to anthropogenic stress. It is well known that urban pollution increases oxidative stress - a state when the antioxidant system is overwhelmed by oxidants, which may cause tissue damage linked to disease and senescence. Urban great tits (Parus major) have, repeatedly, shown to have a higher antioxidant activity than rural conspecifics, sometimes sufficient enough to prevent tissue damage and sometimes not. Regardless, an increased antioxidant defence is probably not cost free for urban birds, which is why we need to understand the underlying mechanisms. Is it a result of: i) the evolutionary history (i.e. a strong selection pressure for greater antioxidant capacities in urban habitats); ii) the present environment (i.e. direct physiological up-regulation in response to current urban stressors); or iii) the individual history combined with the present environment (i.e. developmental programming of gene regulation and the potential for match/mis-match between environment and optimal physiological response). By using the European great tit as our model system, my group address these three mechanistic pathways for generating variation in antioxidant capacities between populations and individuals.