Artificial light at night disrupts the navigation, timing, and behavior of migratory birds, contributing to collision mortality and the energetic costs of disorientation during long journeys.
Migratory bird welfare during urban transit is a scale problem: billions of individual birds make twice-annual journeys through increasingly lit human environments, and the disorientation, collision mortality, and energetic costs this imposes are enormous. Individual birds that collide with lit buildings suffer acute trauma and death. Lights-out programs provide a direct welfare intervention that reduces this mortality without restricting human activity significantly — most building lighting during migration is unnecessary and unused. The welfare benefit per unit of effort is unusually high: switching off lights overnight in migration season directly reduces billions of mortality events across the cumulative population.