Assessing the combined threats of artificial light at night and air pollution for the world’s nocturnally migrating birds

Aim Two important environmental hazards for nocturnally migrating birds are artificial light at night (ALAN) and air pollution, with ambient fine particulate matter (PM2.5) considered to be especially harmful. Nocturnally migrating birds are attracted to ALAN during seasonal migration, which could increase exposure to PM2.5. Here, we examine PM2.5 concentrations and PM2.5 trends and the spatial correlation between ALAN and PM2.5 within the geographical ranges of the world’s nocturnally migrating birds. Location Global. Time period 1998–2018. Major taxa studied Nocturnally migrating birds. Methods We intersected a global database of annual mean PM2.5 concentrations 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 PM2.5 concentrations and trends and measured the correlation between ALAN and PM2.5, which we summarized by season and flyway. Results Correlations between ALAN and PM2.5 were significantly positive across all seasons and flyways. The East Asia–Australia flyway had the strongest ALAN–PM2.5 correlations within regions of passage, the highest PM2.5 concentrations across all three seasons and the strongest positive PM2.5 trends 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. Main conclusions 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 PM2.5 trends in the East Asia–Australia flyway and maintaining negative PM2.5 trends in the Americas and Africa–Europe flyways while reducing ALAN levels would likely be beneficial for the nocturnally migrating bird populations in each region.