Artificial light and its effect on bird migration: insights from concurrent acoustic and radar monitoring

For birds that migrate nocturnally, starlight plays a critical role in navigation; however, light from anthropomorphic sources can disrupt navigation. These disruptions could result in losses of biodiversity if anthropogenic lights have detrimental effects on migrating birds. In order to prevent continued biodiversity losses of migratory animals, careful analyses of artificial light disruptions are required. Radar tracking and acoustic monitoring have been used previously in the study of avian migration, but these techniques have yet to be used concurrently in studying migratory disruption by artificial light. In this study, I used a combination of acoustic and radar techniques to understand whether avian migration is disrupted by anthropogenic light. I used NEXRAD radar to estimate bird traffic rates and altitudinal positioning of migrating birds, and acoustic monitoring to identify the species of those migrants. I used these techniques in tandem to study birds passing over sites with artificial light and nearby dark control sites. NEXRAD data provided evidence of birds passing over artificially lit areas in greater numbers and altitudinal changes in migrating flocks. Acoustic data provided evidence of increased call rates and changes in flock composition, suggesting increased disorientation and reduced cohesion of natural mixed species flocks near artificial light. I conclude that artificial light changes migratory behaviour of birds, imposing greater energetic costs to migration. This research, and parallel studies of anthropogenic light, highlight the importance of understanding the effect of anthropogenic light, as expanding urbanization removes remaining unaltered refugia and natural biological processes become increasingly disrupted by human influences