Artificial light at night weakens body condition but does not negatively affect physiological markers of health in great tits.

Urbanisation brings many novel challenges for wildlife through changes to the natural environment; one of the most unprecedented of these modifications is artificial light at night (ALAN). ALAN has been shown to have profound effects on the behaviour and physiology of many wildlife species, which in turn have negative consequences for fitness and survival. Despite increasing knowledge of the mechanisms by which ALAN can affect health, studies that have investigated this relationship have found contrasting results. This study investigated the impact of ALAN on health biomarkers in 13 day old great tit (Parus major) nestlings including malondialdehyde levels (a measure of oxidative damage), antioxidant capacity of plasma, feather corticosterone levels and scaled mass index. Immediately after hatching, broods were either exposed to 1.8 lx of ALAN until day 13 or left unexposed. ALAN treatment significantly reduced scaled mass index but there were no clear negative effects of ALAN on malondialdehyde levels, antioxidant capacity or corticosterone levels. This demonstrates that only certain aspects of health are impacted by early-life ALAN, highlighting the importance of future studies measuring several biomarkers of health when investigating this relationship. Nestlings that fledge the nest in poor body condition have a decreased chance of surviving into adulthood. As urbanisation continues to expand, the negative effects of ALAN on wildlife are likely to become more pronounced. Therefore, it is crucial to gain a better understanding of this relationship.