Artificial light at night disrupts the navigation, timing and rest of migratory birds, causing millions of collision deaths and population-level disruption to migration phenology across continents.
Light pollution represents a diffuse, invisible and largely unintentional source of mass bird mortality and suffering. Birds drawn off course by urban light sources circle illuminated buildings until exhausted, colliding with glass or each other. Those that survive the attraction event are often so disoriented and depleted that they cannot resume migration effectively. Light pollution also disrupts the biological timing systems that coordinate migration departure, breeding and moult — potentially misaligning these events with food availability windows in ways that reduce survival and reproductive success. Relatively simple interventions — turning off non-essential lighting during spring and autumn migration peaks, using bird-safe red-spectrum lighting and treating glass surfaces — can dramatically reduce individual suffering and population-level disruption.