earlylife exposure to artificial light at night elevates physiological stress in freeliving songbirds artificial light at night alan can disrupt adaptive patterns of physiology and behavior that promote high fitness resulting in physiological stress and elevation of steroid glucocorticoids corticosterone cort in birds elevated cort may have particularly profound effects early in life with the potential for enduring effects that persist into adulthood research on the consequences of earlylife exposure to alan remains limited especially outside of the laboratory and whether light exposure affects cort concentrations in wild nestling birds particularly remains to be elucidated we used an experimental setup to test the hypothesis that alan elevates cort concentrations in developing freeliving birds by exposing nestling great tits parus major to alan inside nest boxes we measured cort in feathers grown over the timeframe of the experiment 7 nights such that cort concentrations represent an integrative metric of hormone release over the period of nocturnal light exposure and of development we also assessed the relationships between feather cort concentrations body condition nestling size rank and fledging success in addition we evaluated the relationship between feather cort concentrations and telomere length nestlings exposed to alan had higher feather cort concentrations than control nestlings and nestlings in poorer body condition and smaller brood members also had higher cort on the other hand telomere length fledging success and recruitment rate were not significantly associated with light exposure or feather cort concentrations results indicate that exposure to alan elevates cort concentrations in nestlings which may reflect physiological stress in addition the organizational effects of cort are known to be substantial thus despite the lack of an effect on telomere length and survivorship elevated cort concentrations in nestlings exposed to alan may have subsequent impacts on laterlife fitness and stress sensitivity