Billions of migrating birds take flight across North America each fall and spring, heading south to their wintering grounds or north to their breeding grounds. 

Most of these birds are songbirds and they’re on the move at night. At times during their long migration, they need to stop, rest and refuel before they take off again. But instead of landing in their typical habitats such as forests or wetlands, artificial light is drawing them within and around cities. 

That’s a problem, said Jeff Buler, a University of Delaware professor of wildlife ecology, because light pollution can be an “ecological trap” for birds. 

“It lures them into cities where they’re at greater risk of colliding with buildings or mortality from other sources like feral cats,” Buler said. 

New research published in the journal Nature Communications finds that skyglow — how bright the night sky is because of artificial light — is a top predictor of where large numbers of migrating birds are going to stop and rest during their migrations across the U.S.

The study, which uses bird stopover data from 2016 to 2020, is a collaboration between UD’s Buler and researchers at Colorado State University, the National Park Service, the Cornell Lab of Ornithology, Princeton University, University of Massachusetts Amherst and Michigan State University. The research was supported with funding from NASA and the National Science Foundation.

Scaling up 

Researchers have known for a while that migrating birds are attracted to artificial light in the night sky. They’ve seen this phenomenon in the Northeast U.S., the Gulf of Mexico and the Great Lakes region. 

Now they have evidence that it’s happening at a nationwide scale, across the contiguous U.S.

How? By using the same technology that meteorologists use to forecast the weather.

Buler helped design the study, which builds off of a data processing algorithm he developed a decade ago. The researchers looked at weather surveillance radar scans of the sky around evening twilight when birds start their migratory flights. Radars around the country detected the largest swaths of migrating birds in the air about an hour after sunset.

“We’re using the radar images very close to the onset of these nocturnal flights,” Buler said. “Birds are very well synchronized in the timing of when they take off at night. Those first couple of images when birds start to leave their stopover sites on the ground and fly up into the radar imagery gives us an idea of where they are in the landscape and at what densities.”

The researchers analyzed more than 3 million weather radar scans to get a picture of how birds are distributed across the U.S. at night. For both fall and spring migrations, they found skyglow to be one of the biggest factors driving larger numbers of birds to stop and stay somewhere along their migration. They also found elevation, distance to radar, precipitation and year to be four other big predictors.  

Buler said migrating birds’ attraction to light seems to be stronger along the Pacific Flyway, compared to other parts of the U.S. The research team had never studied birds’ attraction to artificial light along the Pacific Flyway before.  

Kyle Horton, an assistant professor at Colorado State University’s Department of Fish, Wildlife and Conservation Biology and the lead author on the paper, said the study is the culmination of research from the last 10 years leading to the first U.S.-wide map showing birds’ stopover sites based on weather radar data. 

“It’s really exciting for us to get to this stage to be able to deliver it to land managers and conservation managers who can use the data in that capacity,” said Horton, an alumnus of the UD College of Agriculture and Natural Resources.

‘Philosophical challenges’ for conservation  

With a broader perspective on how widespread this problem is for migrating birds across the U.S. Buler said he hopes the research team’s findings will inform conservation efforts to protect migratory birds. 

“The scope of the impact is kind of shocking,” Buler said. “Historically, we've known that birds are drawn to lights at a very localized scale. But recognizing that this really scales up to where cities are acting like beacons to these birds and drawing them in from over a hundred kilometers away into these urbanized areas raises concern for population-scale impacts.”

It’s still a mystery why migrating birds are attracted to artificial light. There’s questions on how to study that. But weather radar may not be the tool that’s needed. 

Ecologically, it makes sense for birds to migrate at night, Horton said. Migrating at night helps birds avoid predators, temperatures are cooler, and the atmosphere is not as turbulent as it is during the day. 

But Horton said the paper’s findings present a sort of conservation dilemma. If many birds aren’t stopping at wetlands or forests as they normally would and they’re stopping to refuel in or near cities instead, how do scientists protect those areas? 

“We might say, ‘We want to protect the hotspots birds are using for stopover,’” Horton said, acknowledging protecting the areas that a consistently large number of birds use to stop and rest. “And one of the hotspots that we have is driven by light pollution. Should we go ahead and protect that area, which might be bad for birds, but we know they're using that area? Do we focus on protecting that area or do we really focus on trying to reduce light pollution?” 

In a new paper in Current Biology, UD’s Buler, a co-author, used weather radar data to look at the stopover sites migrating birds are using along their fall and spring migrations in the eastern U.S. Researchers combined maps of birds’ stopover sites in spring and fall together to get a broader picture of where birds are stopping. Then they overlaid a map of protected areas from the U.S. Geological Survey to their combined stopover map to assess how well protected birds’ stopover sites are. The research was funded by the High Meadows Foundation and the U.S. Department of Agriculture.

The findings from the Nature Communications paper show artificial light is drawing large numbers of migrating birds near cities across the U.S. Consequently, the Current Biology paper shows that forest fragments in regions where few forests occur have the highest chance of being a popular resting place for birds. 

They also found only about one-third of the hotspots have some sort of a protection status, meaning they’re preserved and can’t be developed.

“While protected areas like state wildlife management areas and national forests are critical for migratory birds, our study also revealed that more than two-thirds of the most important stopover hotspots are unprotected areas at risk of disappearing in the future,” Buler said.

Ultimately, the researchers say more conservation efforts are needed to protect more of the places birds are stopping at on their migrations, so migrating birds can continue to thrive.  

The paper “Artificial light at night is a top predictor of bird migration stopover density” was authored by Kyle Horton of Colorado State University, Jeff Buler of University of Delaware, Sharolyn Anderson of National Park Service, Carolyn Burt of Colorado State University, Amy Collins of Colorado State University, Adriaan Dokter of Cornell University, Fengyi Guo of Princeton University, Daniel Sheldon of University of Massachusetts Amherst, Monika Anna Tomaszewska of Michigan State University, and Geoffrey Henebry of Michigan State University. 

The paper “Seasonal patterns and protection status of stopover hotspots for migratory landbirds in the eastern United States” was authored by UD’s Jeff Buler and Jaclyn Smolinsky as well as Princeton University’s Fengyi Guo and David Wilcove.