Artificial lights at night (ALAN) have multiple effects on night-migrating birds due to positive phototaxis. Flight paths of migrating birds can be curved or circular rather than straight near bright beams of light aimed skywards, and near lights on communication towers. ALAN installations like these, however, are uncommon. For example, luminaries on communication towers account for ~0.1% of outdoor lighting in the US, while parking lights account for >30%. To test whether ground-based, downwards-pointing lights also affect the flight behavior of migrating birds, we installed three LED spotlights (22000-Lumen each) pointing ~45° downward on top of 5m poles in a dark, sparsely lit rural landscape. We turned these lights on and off sequentially throughout three consecutive bird migration seasons. During ~70 nights we collected >1800 high-resolution 3D flight paths, up to 5km long, of free-flying nocturnally migrating birds using a tracking radar. We predict that low flight altitudes and proximity to our downcast experimental lights relate to a greater probability of behavioral reactions during flight. Nocturnal migrants in North America overfly cities and many other areas with widespread downcast lights. This work will inform us if the most common but often dismissed ALAN installations affect the flight behavior of nocturnally migrating birds.
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