by Light pollution has steadily increased and expanded from urban areas, and with the advent of LED lighting, it is growing in North America by up to 10% per year, as measured by the visibility of stars in the night sky. In our recent study, we found that the glow from cities and urban fringes can be a powerful attraction for migratory birds, drawing them into developed areas where food is scarcer and they face threats such as collisions with glass buildings.
Every spring and fall, migratory birds move to or from the breeding grounds, sometimes traveling thousands of miles. On the way, most birds have to stop to rest and feed. Some species burn half of their body mass during migration.
Migration stopover locations are not random, and birds tend to use the same locations from year to 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 attracts birds to these locations.
We found that light pollution was a top predictor of migratory bird density at stopover sites for both spring and fall migration across the continental US.
Why It Matters
Almost all birds in North America – about 80% – migrate each spring and fall. And of those species that migrate, 70% travel at night.
Nocturnal migration has many adaptive advantages: For example, the weather conditions are better, and fewer predators are active. But it puts most migratory birds at risk of light pollution. In North America alone, it is estimated that up to 1 billion migratory birds die each year due to collisions with buildings.
Scientists still don’t know why nocturnal migratory birds are drawn to artificial light, but research has shown that light pollution acts as an amplifying agent that draws more canines into urban areas. It often occurs with other environmental threats, such as water and air pollution and noise. All these pressures affect the birds’ behavioral and physiological processes during an already very taxing journey.
Lighting is part of the fabric of human structures, but many people do not think of it as a pollutant or realize its harmful effects on nature – until events such as the mass loss of birds in Chicago on 4-5 October, 2023, when it was almost. 1,000 birds were killed after a collision with the McCormick Place Convention Center, making the problem impossible to ignore.
How we did our work
Together with colleagues at Colorado State University, Michigan State University, University of Delaware, Cornell Laboratory of Ornithology, Princeton University, University of Massachusetts Amherst and the National Park Service, we sought to understand the complex drivers and large-scale patterns that is about change. density by combining remote sensing data with geospatial tools. Mapping stopover sites has been a priority for bird conservation for many years; now, for the first time, we have a complete view of where these stops are across the United States.
We were able to make novel maps on a continental scale using US NEXRAD weather surveillance data – information from the same radars that meteorologists use to predict weather patterns on TV and weather apps. We created 2,500 models using about 1 million locations across the US and 49 predictor variables, including forest cover, precipitation, temperature, elevation and skylight—diffuse brightness in the night sky from artificial light.
These maps capture fine-scale detail that allows us to see the increased densities of migratory birds following the winding banks of the Mississippi River, providing an important refuge for exhausted migrants to rest and refuel. We also created maps of autumn and spring hotspots which highlight the regions where a particularly high number of birds made stopovers.
We found that the presence of light pollution was a better predictor of bird densities than temperature, precipitation or tree canopy cover. These were all variables that we expected to correlate with periods when birds were on the ground, or with quality habitats where birds were likely to stop again.
Other variables related to areas that birds were unlikely to use as stopovers. One example was the presence of agricultural crops, such as corn or soybeans. Single-crop fields do not provide enough food or shelter for many bird species, so migrants are unlikely to nest there.
Light pollution is a human-made change to the environment that can act as an ecological trap, drawing birds into substandard habitats and increasing the risk of collisions with buildings. Happily, its immediate effects can be quickly reversed with the flip of a switch.
By working to reduce artificial light through Lights Out campaigns and migration alerts, bird deaths from light pollution will be reduced by understanding when birds are in airspaces and using bird-friendly glass with cross-patterned surfaces to make it more visible to birds. Understanding the drivers and macro-scale patterns of stand densities across the continental US will better inform conservation actions like these.
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This article is republished from The Conversation, a non-profit, independent news organization that brings you reliable facts and analysis to help you make sense of our complex world. 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, advise, share in or receive funding from any company or organization that would benefit from this article, and does not she disclosed any relevant connections beyond their academic appointment.
