Artificial light attracts migratory birds to cities, where they face a range of threats

Light pollution from urban areas has steadily increased and expanded, and with the advent of LED lighting, it is growing by as much as 10% per year in North America, as measured by the visibility of stars in the night sky. In our recent research, we found that the glow of cities and urban suburbs can strongly attract migratory birds, luring them to developed areas where food is scarcer and exposing them to threats such as collisions with glass buildings.

Every spring and fall, migratory birds travel to or from their breeding grounds, sometimes traveling thousands of miles. Along the way, most birds have to make stops to rest and eat. Some species burn half their body mass during migration.

Stopover sites are not random and birds typically use the same locations year after year. Because migration occurs on a continental scale, with billions of birds crossing North America each migration season, it is important for scientists to understand what draws birds to these locations.

We found that light pollution was a significant predictor of migratory bird density at stopover locations for both spring and fall migration across the continental US.

Why it matters

Nearly all birds in North America – about 80% – migrate every spring and fall. And of the species that migrate, 70% travel at night.

Nocturnal migration has many adaptive advantages: for example, weather conditions are better and fewer predators are active. But it makes most migratory birds very sensitive to light pollution. In North America alone, it is estimated that up to 1 billion migratory birds die each year from collisions with buildings.

Scientists don’t yet know why night-migrating birds are attracted to artificial light, but research has shown that light pollution acts as a tonic, attracting more songbirds to urbanized areas. It often comes together with other environmental threats, such as water and air pollution and noise pollution. All these stressors disrupt the behavioral and physiological processes of birds during journeys that are already extremely stressful.

Lighting is part of the fabric of human structures, yet many people do not consider it a pollutant or see its harmful effects on nature – until events such as the massive bird loss in Chicago on October 4 and 5, 2023, when nearly 1,000 birds were killed after a collision with the McCormick Place Convention Center, making the problem impossible to ignore.

A black bird with an orange underside sits on a branch next to half an orange placed there for eating.

How we did our work

With colleagues from Colorado State University, Michigan State University, the University of Delaware, the Cornell Lab of Ornithology, Princeton University, the University of Massachusetts Amherst, and the National Park Service, we sought to understand the complex factors and large-scale patterns of stopover density by combining remote sensing data with geospatial tools. Mapping stopover locations has been a bird conservation priority for years; now, for the first time, we have a complete picture of where these stops are in the United States.

We were able to create new continental-scale maps using U.S. NEXRAD weather surveillance data – information from the same radars that meteorologists use to predict weather patterns on television and weather apps. We created 2,500 models using approximately 1 million locations across the US and 49 predictor variables, including forest cover, precipitation, temperature, altitude, and skyglow – diffuse brightness in the night sky due to artificial lighting.

These maps capture fine-scale details that allow us to see a greater density of migratory birds following the winding banks of the Mississippi River, which provide an important refuge for exhausted migrants to rest and refuel. We also created fall and spring hotspot maps focusing on regions where especially large numbers of birds were stopping over.

Radar images show masses of light and dark blue above a map of St. Louis.Radar images show masses of light and dark blue over a map of St. Louis.

We found that the presence of light pollution was a better predictor of bird density than temperature, precipitation or canopy cover. These were all variables that we expected to correlate with periods when birds would be on the ground, or with high-quality habitats where birds were likely to stop over.

Other variables were related to areas that birds were unlikely to use as stopovers. An example was the presence of agricultural crops, such as corn or soybeans. Fields planted with a single crop do not provide sufficient food or shelter for many bird species, so migrants are unlikely to rest there.

Light pollution is a human-induced environmental change that can act as an ecological trap, attracting birds to substandard habitats and increasing the risk of collisions with buildings. Fortunately, its immediate effects can be quickly reversed with the push of a button.

By working to reduce artificial light through Lights Out campaigns and migration alerts, by understanding when birds will be in air spaces and by using bird-friendly glass with patterns across its surface to make it more visible to birds, bird mortality due to light pollution will be reduced. Understanding the macroscale drivers and patterns of stopover density in the continental US will better inform these types of conservation actions.

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This article is republished from The Conversation, an independent nonprofit organization providing facts and trusted analysis to help you understand our complex world. It was written by: Carolyn S. Burt, Colorado State University and Kyle Horton, Colorado State University

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Kyle Horton receives funding from the National Science Foundation and the National Aeronautics and Space Administration.

Carolyn S. Burt does not work for, consult with, own stock in, or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.

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