Fatal collisions with built structures represent a major source of direct, human-caused bird mortality across North America, second only to predation by domestic cats ( 2). North America has lost nearly one-third of its birdlife in the last half-century, with migratory species experiencing particularly acute declines ( 1). Although our research focuses on a single site, our findings have global implications for reducing or eliminating a critically important cause of bird mortality. Our study provides strong support for a relationship between nocturnal migration magnitude and urban bird mortality, mediated by light pollution and local atmospheric conditions. Bird mortality could be reduced by ∼60% at this site by decreasing lighted window area to minimum levels historically recorded. We estimate that halving lighted window area decreases collision counts by 11× in spring and 6× in fall. The greatest mortality occurred when the building was brightly lit during large nocturnal migration events and when winds concentrated birds along the Chicago lakeshore. We find that the magnitude of nocturnal bird migration, building light output, and wind conditions are the most important predictors of fatal collisions. Using two decades of collision surveys and concurrent weather and migration measures, we model numbers of collisions occurring at a large urban building in Chicago. Reducing this source of mortality requires knowledge of important behavioral, meteorological, and anthropogenic factors, yet we lack an understanding of the interacting roles of migration, artificial lighting, and weather conditions in causing fatal bird collisions. Millions of nocturnally migrating birds die each year from collisions with built structures, especially brightly illuminated buildings and communication towers.
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