Nighttime is a critical window for many bird species during their migrations in spring and autumn, but it also plays a role in non-migratory periods. A recent study focusing on the Americas, with insights applicable to Europe, shows that rising light pollution is startling these birds and altering their behavior.
Light has a dual effect on birds: it attracts and disorients them. This misdirection can lead to injuries or even fatalities when birds collide with windows and buildings. Because of these risks, birds tend to avoid bright illumination during the breeding season and at other times of the year as well.
Yet during migration, birds cross vast landscapes that include urban areas where light pollution cannot always be avoided. In southeastern parts of the United States, Mexico, and Central America—regions that serve as important migration corridors—the extent of light-polluted land has risen to about 16% over the last twenty years.
Birds need darkness at night for safe navigation and rest. In the Americas, only a small share of land has lower light levels, a situation linked to shrinking bird populations and urban heat effects in places like the northeastern United States. A new study in Ecosphere details this trend and its implications for night-flying birds during both migration and non-migration periods.
Scientists at the Cornell Lab of Ornithology and Colorado State University emphasize that this shift poses a real threat to nocturnal migrants during spring and autumn and even during other seasons.
Lead author Frank La Sorte of the Cornell Lab highlights the alarming evidence that nocturnal migratory birds are encountering higher levels of artificial light in these regions. This assessment draws on data collected by experts and collaborators and is reported by researchers affiliated with prominent institutions.
Turn off the lights during transitions
The study integrates data from eBird, a citizen-science program run by Cornell Labs, where volunteers contribute their observations from around the world to help generate weekly estimates of relative bird abundance. In total, 42 species were included in the analysis.
Researchers compared these abundance estimates with twelve months of satellite measurements of light pollution gathered over a 22-year span.
The findings show that every species examined occurred in areas of elevated light pollution. The most pronounced increases appeared during migration, especially for birds traveling through Central America, while the smallest increases were observed in species that breed in the Northeastern United States during the summer.
A persistent trend noted by researchers involves stronger light-pollution effects in Central America during the spring. To address this, some programs promote Lights Out initiatives during transitional periods to reduce exposure for migratory birds.
The researchers also propose applying similar considerations to the southeastern United States and Central America during the non-breeding winter season. The aim is to maximize benefits outside migratory windows and to lessen risks when birds are most vulnerable.
This work helps scientists map where and when birds will encounter current or increasing light pollution, enabling more targeted efforts to reduce risk and improve survival for these species.
Reference work: https://esajournals.onlinelibrary.wiley.com/doi/full/10.1002/ecs2.3994