Cities Matter More to Bird Migration Than We Thought

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Key takeaways: urban bird migration 

• A recent study found that nearly half of all U.S. migration hotspots, where many birds rest and refuel, are in metropolitan areas. 
• This could be because cities often feature more water and artificial lights. 
• The trend was clearer in the western United States, especially in higher-income areas. 
• Researchers used weather radar data to track migration patterns on an unprecedented scale.
• To help migrating birds safely navigate urban spaces, people can turn off lights at night and make windows bird-safe.  

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When Miguel Jimenez first saw his study’s results, he thought they were a mistake. They showed millions of migrating birds resting and refueling in urban America on a huge, unexpected scale.

Scientists have largely overlooked the role of cities in bird migration, says Jimenez, who conducted the study as part of his Ph.D. at Colorado State University. “There’s this narrative that on the conservation front, there’s not much action that we can take in cities,” he says. But he didn’t believe it. “I wanted to know how much migratory stopover actually happens in cities.”

The answer: A lot. The study, published earlier this year in Nature, used weather radar data to analyze bird migration stopover patterns across the continental United States. The team found that nearly half of the “hotspots”—places where especially high numbers of birds rest and refuel—are in metropolitan areas: 48 percent of them in spring, and 44 percent in fall. “Metropolitan areas,” here, are defined areas with an urban core of at least 50,000 inhabitants, plus the surrounding communities. Given their relatively limited land area, that makes these places disproportionately popular with birds. And since 86 percent of Americans live in them, most of us can help birds passing through. 

“The initial hotspot numbers were pretty shocking, but the more I thought about it, it made a lot of sense,” Jimenez says. Why? He has two major theories: Light and water. Cities are immense sources of artificial light, which attracts birds, especially at night (and often with deadly consequences). Equally, cities are often built along waterways, or engineered to bring water in, drawing in migrants in need of water and food. Supporting that theory, the study found more urban hotspots in the western United States than in the East—perhaps because western cities provide more resources than their arid surroundings. “The West, in a lot of ways, is a big desert,” says Melanie Smith, science and conservation product director at Audubon, who was not involved in the study. 

Water might also explain another of the study’s findings: In arid regions, more birds were found in higher-income areas. The researchers say this is evidence of the luxury effect, which predicts that biodiversity tends to increase in wealthier areas. That effect was clearest in drier western cities like Los Angeles, Phoenix, and San Diego, where affluent areas may offer more water and vegetation. “People and birds alike prefer wetter, greener spaces,” says Smith, who led development of Audubon’s Bird Migration Explorer. “Urban green spaces in these western cities aren’t just recreational amenities for people. They’re functioning as habitat.”
 

To date, most studies of urban birds have focused on a single city or region, limiting insights into cities’ roles in major ecological processes like migration. But by zooming out, researchers can better spot patterns across diverse cities and landscapes—and, in turn, identify widely applicable ways to make cities more bird-friendly.

How Weather Radar Aids Bird Migration Science and Conservation 

Weather radar helps unlock that possibility, the study shows. Just as radar waves detect weather like rain and snow, they also pick up flocks of birds flying through the air—and while radar can’t sense individual birds, it can provide estimates of the number of birds in an area.Increasingly used in bird migration science, radar tools like Cornell’s BirdCast offer more comprehensive, impartial data than other observation methods. Community science databases like eBird and iNaturalist can be skewed by social inequity, for example: People contributing to community science are more likely to live in and report from affluent areas, which can distort data about where wildlife is most abundant. 

“That’s what’s most unique and exciting about radar,” says Jimenez, who was formerly an outreach biologist for Audubon’s Migratory Bird Initiative. “We’re getting a relatively even sampling of where there are birds.” Radar isn’t perfect—it can’t distinguish between species, for example. But combined with other data sources, it can be illuminating. Smith agrees: “Through sources like eBird, you get a sense of the species mix and relative abundance of those species. By pairing that with radar’s estimate of abundance, you get a powerful tool for monitoring urban areas.” 

After all, the idea that migrating birds can converge, en masse, in cities isn’t new: Cities on major migration paths have long been seen as hubs for birds and birders alike. The parks of New York City, on the Atlantic Flyway, are a classic example. But the unprecedented scale of this study, enabled by radar, reveals just how widespread the phenomenon is, and sheds light on nuances within it. 

Ultimately, the study speaks to how quickly the country is still urbanizing, Jimenez says. Studies like his show how we can make urban development better and safer for birds—and since so many birds rest in cities, it’s critical that we do so. Each year in the United States, over one billion birds are killed by collisions with windows, especially during migration. That number is rising as urbanization continues. Cities’ light pollution can attract and disorient birds that navigate by starlight, pulling them off course. And by reducing green space to small, isolated patches, urban sprawl leaves tired birds with limited food and safe places to rest. 

But there are proven ways that people can help. To support migrating birds, citydwellers can turn off lights and install bird-safe glass, keep cats indoors, set up bird feeders, and plant native plants or microforests. Everyone can advocate for local and federal policy changes, like the proposed federal Bird-Safe Building Act; some regional governments, from Minnesota to New York City, have adopted bird-safe policies, but many more are needed. People can also support urban rewilding projects and programs like Audubon’s Lights Out, a national effort to get building owners, managers, and residents to cut artificial light at night. 

For the birds journeying through our urban centers, any one of these steps—big or small—could be the difference between a successful migration and a tragic ending.