When nocturnally migrating birds encounter low-level light pollution patches: a case study from the Croatian coast

Artificial light is a pollutant of growing global concern. For nocturnally migrating birds, the consequences can be fatal. Attracted and disoriented by illuminated infrastructure, birds can become victims of collisions, especially when visibility is reduced by fog and clouds. Birds crossing large, predominantly dark bodies of water can suddenly be confronted with patches of lit coastal areas. In contrast, when flying over land along the coast, birds are sequentially confronted with lit areas and are likely to rely on different navigation cues. We deployed two ornithological radars in proximity along the Croatian coast: one at a light-polluted site and one at a near-natural site. The aim was to maximise the contrast in light pollution while keeping other site-specific factors similar. We monitored the consecutive spring migration seasons of 2023 and 2024 and modelled the effect of light pollution on the number of birds in the air, mean airspeeds and mean flight altitudes, considering atmospheric, temporal, and directional predictors. Our results are partially hampered by the fact that we had to exclude a second radar pair from the analysis due to a technical defect. Nevertheless, we found evidence for attraction towards light pollution of sea-crossing birds in the remaining radar pair. Furthermore, we found a significant contribution of light pollution to the reduction of mean airspeeds and altitudes, especially in an overcast context. Besides indicating disorientation, our results raise serious concerns about increased numbers of bird collisions associated with migration peaks and impaired visibility, even with lower-intensity light pollution. • We analysed the in-flight reaction of avian migrants to low-level light pollution. • Migration intensity was elevated in sea-crossing birds at the light-polluted site. • Flight altitudes and airspeeds were reduced at the light-polluted site. • Poor visibility was linked to even lower altitudes, raising the risk of collisions. • Calling for action, as even low-level light pollution may affect migratory birds.