Kyle G. Horton

Aim A unique risk faced by nocturnally migrating birds is the disorienting influence of artificial light at night (ALAN). ALAN originates from anthropogenic activities that can generate other forms of environmental pollution, including the emission of fine particulate matter (PM 2.5 ). PM 2.5 concentrations can display strong seasonal variation whose origin can be natural or anthropogenic. How this variation affects seasonal associations with ALAN and PM 2.5 for nocturnally migrating bird populations has not been explored. Location Western Hemisphere. Time Period 2021 Major Taxa Studied Nocturnally migrating passerine (NMP) bird species. Methods We combined monthly estimates of PM 2.5 and ALAN with weekly estimates of relative abundance for 164 NMP species derived using observations from eBird. We identified groups of species with similar associations with monthly PM 2.5 . We summarized their shared environmental, geographical, and ecological attributes. Results PM 2.5 was lowest in North America, especially at higher latitudes during the boreal winter. PM 2.5 was highest in the Amazon Basin, especially during the dry season (August–October). ALAN was highest within eastern North America, especially during the boreal winter. For NMP species, PM 2.5 associations reached their lowest levels during the breeding season (<10 μg/m 3 ) and highest levels during the nonbreeding season, especially for long‐distance migrants that winter in Central and South America (~20 μg/m 3 ). Species that migrate through Central America in the spring encountered similarly high PM 2.5 concentrations. ALAN associations reached their highest levels for species that migrate (~12 nW/cm 2 /sr) or spend the nonbreeding season (~15 nW/cm 2 /sr) in eastern North America. Main Conclusions We did not find evidence that the disorienting influence of ALAN enhances PM 2.5 exposure during stopover in the spring and autumn for NMP species. Rather, our findings suggest biomass burning in the Neotropics is exposing NMP species to consistently elevated PM 2.5 concentrations for an extended period of their annual life cycles.