Effects of Light Pollution on Activity Patterns and Clocks of City and Forest Birds

Most organisms exhibit 24-hour rhythms in their behaviour and physiol<br/>ogy, synchronized to their habitat’s light/dark (LD) cycle. Artificial light at<br/>night (ALAN) disrupts the natural LD cycle, affecting behaviour and physiology across taxa. In wild birds, the effect of ALAN is shown to be weaker in city birds, but whether this difference is genetic or due to plasticity remains unclear. To investigate this question, we conducted a common garden experiment<br/>using great tits (Parus major) from city and forest populations to test: 1) whether sensitivity to ALAN differs genetically between city and forest birds, 2)<br/>whether responses to ALAN are linked to circadian clock properties or direct masking effects, and 3) whether city and forest birds differ in dose-dependent responses to light. For objectives 1 and 2, first-generation (F1, i.e. birds raised<br/>by hand from eggs originating from the wild) birds were exposed to either ALAN or a dark control in climate-controlled aviaries across breeding season in subsequent years with reversed treatments. Daily foraging activity patterns were obtained through a RFID feeder system. Circadian clock properties were subsequently assessed in constant dim light conditions and via light pulse experiments. City birds showed reduced sensitivity to ALAN in activity onset compared to forest birds. This behavioural difference was not linked to circadian clock differences, suggesting ALAN influences behaviour without affecting theclock. For objective 3, third-<br/>generation (F3) birds were exposed to varying ALAN intensities (0.5, 1.5, 5 lux) to<br/>explore potential differences in dose-dependent masking effects. Both<br/>groups showed similar dose-dependent responses, suggesting reduced sensitivity in city birds may reflect environmental plasticity, or a difference in acclimation to lo<br/>ng-term ALAN exposure. This study demonstrates how ALAN affects daily activity patterns in birds and provides insights into the potential evolutionary and ecological consequences of human-induced environmental changes.