Experimental exposure of indirect light at night alters western bluebird parent activity and nestling development

The pervasive spread of artificial light at night has been documented to disrupt natural rhythms with varying consequences on wildlife. Disruption to the night sky can alter nestling development either through indirect exposure to light or due to changes in adults' behaviour, both having potential physiological costs or benefits. To test these effects, we experimentally manipulated light outside of western bluebird, Sialia mexicana , nesting cavities and investigated whether exposure to light at night affects nestling development and adult behaviour. We found no evidence of light at night advancing or delaying the onset or cessation of adult daily activity; however, repeatability analyses suggested that onset and cessation of daily activity was more repeatable for pairs breeding in control nests compared to those in nests with experimental light exposure. Additionally, we found that nestlings exposed to light had smaller wing chords and lower mass than those in the control. The effect of light on night was dependent on brood size: nests exposed to light with brood sizes of three nestlings had smaller wing chords than nests without lights, but there were no strong differences between light-exposed nests and control nests with five chicks. Nevertheless, light-exposed chicks were smaller than those in control nests, which could negatively influence fledgling survival. These findings suggest that indirect artificial light outside the nesting cavity can increase variability in daily activity rhythms and influence the development of nestlings. • Light pollution is a globally widespread stimulus affecting birds and other wildlife. • We exposed western bluebird nests to ecologically relevant light levels for 5 days. • Light exposure decreased the repeatability of activity. • Light-exposed nestlings were smaller at the end of the experiment.