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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_2.5concentrations can display strong seasonal variation whose origin can be natural or anthropogenic. How this variation affects seasonal associations with ALAN and PM_2.5for nocturnally migrating bird populations has not been explored.

Western Hemisphere.

2021

Nocturnally migrating passerine (NMP) bird species.

We combined monthly estimates of PM_2.5and 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.

PM_2.5was lowest in North America, especially at higher latitudes during the boreal winter. PM_2.5was 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_2.5associations reached their lowest levels during the breeding season (<10 μg/m³) and highest levels during the nonbreeding season, especially for long‐distance migrants 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.

We did not find evidence that the disorienting influence of ALAN enhances PM_2.5exposure 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_2.5concentrations for an extended period of their annual life cycles.

Two important environmental hazards for nocturnally migrating birds are artificial light at night (ALAN) and air pollution, with ambient fine particulate matter (PM_2.5) considered to be especially harmful. Nocturnally migrating birds are attracted to ALAN during seasonal migration, which could increase exposure to PM_2.5. Here, we examine PM_2.5concentrations and PM_2.5trends and the spatial correlation between ALAN and PM_2.5within the geographical ranges of the world’s nocturnally migrating birds.

Global.

1998–2018.

Nocturnally migrating birds.

We intersected a global database of annual mean PM_2.5concentrations over a 21‐year period (1998–2018) with the geographical ranges (breeding, non‐breeding and regions of passage) of 225 nocturnally migrating bird species in three migration flyways (Americas,n = 143; Africa–Europe,n = 36; and East Asia–Australia,n = 46). For each species, we estimated PM_2.5concentrations and trends and measured the correlation between ALAN and PM_2.5, which we summarized by season and flyway.

Correlations between ALAN and PM_2.5were significantly positive across all seasons and flyways. The East Asia–Australia flyway had the strongest ALAN–PM_2.5correlations within regions of passage, the highest PM_2.5concentrations across all three seasons and the strongest positive PM_2.5trends on the non‐breeding grounds and within regions of passage. The Americas flyway had the strongest negative air pollution trends on the non‐breeding grounds and within regions of passage. The breeding grounds had similarly negative air pollution trends within the three flyways.

The combined threats of ALAN and air pollution are greatest and likely to be increasing within the East Asia–Australia flyway and lowest and likely to be decreasing within the Americas and Africa–Europe flyways. Reversing PM_2.5trends in the East Asia–Australia flyway and maintaining negative PM_2.5trends in the Americas and Africa–Europe flyways while reducing ALAN levels would likely be beneficial for the nocturnally migrating bird populations in each region.

Anthropogenic noise pollution (ANP) is a globally invasive phenomenon impacting natural systems, but most research has occurred at local scales with few species. We investigated continental‐scale breeding season associations with ANP for 322 bird species to test whether small‐scale predictions related to breeding habitat, migratory behaviour, body mass and vocal traits are consistent at broad spatial extents for an extensive group of species.

Conterminous USA.

2004–2011.

North American breeding birds.

We calculated, for each species, the association between the breeding season and ANP, using spatially explicit estimates of ANP from the National Park Service and weekly estimates of probabilities of occurrence based on observations from the eBird citizen‐science database. We evaluated how the association of the breeding season for each species with ANP was related to expectations based on size, migratory behaviour and breeding habitat. For a subset of species, we used vocal trait data for song duration, pitch and complexity to evaluate hypotheses from the birdsong literature related to habitat complexity and sensitivity to ANP.

Species that breed predominantly in anthropogenic environments were associated with twice the level of ANP (~7.4 dB) as species breeding in forested habitats (~3.2 dB). However, we did not find evidence to suggest that birds with higher‐pitched songs are more likely to be found in areas with higher levels of ANP. Residents and migratory species did not differ in associations with ANP, but songs were less complex among forest‐breeding species than non‐forest‐breeding species and increased in complexity with increasing ANP.

Anthropogenic noise pollution is an important factor associated with breeding distributions of bird species in North America. Vocal traits could be useful to understand factors that affect sensitivity to ANP and to predict the potential impact of ANP, although future studies should aim to understand how and why patterns differ across spatial scales.