By Frank La Sorte and Kyle Horton
Millions of birds travel between their breeding and wintering grounds during spring and autumn migration, creating one of the greatest spectacles of the natural world. These journeys often span incredible distances. For example, the Blackpoll warbler, which weighs less than half an ounce, may travel up to 1,500 miles between its nesting grounds in Canada and its wintering grounds in the Caribbean and South America.
For many species, these journeys take place at night, when skies typically are calmer and predators are less active. Scientists do not have a good understanding yet of how birds navigate effectively at night over long distances.
We study bird migration and how it is being affected by factors ranging from climate change to artificial light at night. In a recent study, we used millions of bird observations by citizen scientists to document the occurrence of migratory bird species in 333 U.S. cities during the winter, spring, summer and autumn.
We used this information to determine how the number of migratory bird species varies based on each city’s level of light pollution – brightening of the night sky caused by artificial light sources, such as buildings and streetlights. We also explored how species numbers vary based on the quantity of tree canopy cover and impervious surface, such as concrete and asphalt, within each city. Our findings show that cities can help migrating birds by planting more trees and reducing light pollution, especially during spring and autumn migration.
Declining Bird Populations
Urban areas contain numerous dangers for migratory birds. The biggest threat is the risk of colliding with buildings or communication towers. Many migratory bird populations have declined over the past 50 years, and it is possible that light pollution from cities is contributing to these losses.