Artificial light at night (hereafter "ALAN") affects 88% of the land area in Europe and almost half of the land area in the USA, with even rural areas exposed to lights from agricultural and industrial buildings. To date, there have been few studies that assess the impacts of ALAN on both wildlife behavior and physiology. However, ALAN may alter energy expenditure and/or stress physiology during the breeding period, potentially reducing reproductive success and resulting in conservation implications. Here, we experimentally exposed adult female and nestling tree swallows (Tachycineta bicolor) to ALAN. We then measured the effects of ALAN compared with control conditions on parental behavior (provisioning rate), nestling physiology (corticosterone levels), and reproductive success (likelihood of all eggs hatching and all nestlings fledging per nest). Our results showed that ALAN-exposed females provisioned their nestlings at lower rates than control females. Although relatively weak, our results also suggested that ALAN-exposed nestlings had reduced baseline and increased stress-induced corticosterone compared with control nestlings. ALAN-exposed nestlings also showed greater negative feedback of circulating corticosterone. We found no support for our prediction that ALAN would reduce nestling body condition. Finally, we found some support for a negative effect of ALAN on the likelihood that all eggs hatched in a given nest, but not the likelihood that all nestlings fledged. Therefore, while it is possible that the behavioral and physiological changes found here result in long-term consequences, our results also suggest that direct ALAN exposure alone may not have substantially large or negative effects on tree swallows. Exposure regimes for free-living birds, such as exposure to a combination of anthropogenic disturbances (i.e., ALAN and noise pollution) or direct and indirect effects of ALAN (i.e., effects on physiology due to direct light exposure and alterations in food availability), may produce different results than those found here.
