Biomass burning in the Neotropics is exposing migrating birds to elevated fine particulate matter concentrations

<p><strong>Aim</strong>: 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 originating from natural and anthropogenic processes. How these processes affect seasonal associations with ALAN and PM_2.5 for nocturnally migrating birds has not been documented.</p> <p><strong>Location</strong>: Western Hemisphere</p> <p><strong>Time</strong> <strong>period</strong>: 2021</p> <p><strong>Major taxa studied</strong>: Nocturnally migrating passerine (NMP) bird species</p> <p><strong>Methods</strong>: We combined monthly estimates of PM_2.5 and ALAN with weekly estimates of relative abundance for 164 NMP species within the Western Hemisphere derived using bird observations from eBird. We identify groups of species with shared associations with PM_2.5.</p> <p><strong>Results</strong>: 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 the NMP species, PM_2.5 associations reached their lowest levels during the breeding season (&lt;10 μg/m³) and highest levels during the nonbreeding season, especially for species that winter in Central and South America (~20 μg/m³). Species that migrate through Central America in the spring encountered similarly high PM_2.5concentrations. ALAN associations reached their highest levels for species that migrate (~12 nW/cm²/sr) or spend the nonbreeding season (~15 nW/cm²/sr) in eastern North America.</p> <p><strong>Main conclusions</strong>: 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. </p>