Songbirds generally make their migratory flights at night, and during spring migration tens of millions of birds may be streaming north above us as we sleep. But when the sun rises, where do these tired birds choose to stop, rest, and refuel?
You may picture a nature preserve or grassy field, but a study published earlier this year in the journal Nature Cities shows that a large percentage of these birds are making their “stopovers” in cities, illustrating the importance of urban conservation efforts.
Ornithologist Miguel Jimenez was a Ph.D. candidate at Colorado State University when he led the study as part of his dissertation. The project was inspired by his desire “to do work that was useful to people who are actively working to conserve birds,” he says. “So I had a bunch of conversations with different folks doing that work, and one thing I consistently heard was that it’s often really hard to convince people that bird conservation in cities matters.”
Jimenez’s dissertation focused on studying bird migration using weather radar. Large masses of migrating birds show up clearly on the nationwide radar system used by meteorologists, and this data isn’t subject to the same biases as bird counts carried out by people. If you capture a radar image just as migrating birds are starting out in the morning, Jimenez explains, you can pinpoint the stopover locations from which they’re leaving.
“You see this kind of mushroom cloud of birds taking off, and then they start to dissipate over the landscape.”
Jimenez and his colleagues used data from 143 radar sites to identify stopover hotspots across the continental United States for both spring and fall migration, then calculated how many of those sites fell within urban areas.
“To be totally honest, I ran this analysis originally expecting, like, I’ll probably figure out that most of it doesn’t happen in cities,” says Jimenez.
Instead, nearly half of the stopover sites he found were within what the U.S. Census Bureau has defined as Metropolitan Statistical Areas. Other ways of statistically defining cities showed a similarly disproportionate number of migrating birds using urban stopover sites.
So why would migrating birds choose city habitats?
“Probably a good chunk of my career is [going to be spent] on that question,” says Jimenez.
But there are already some indications. Cities often develop along coastlines and rivers, places that already have high biodiversity, he points out. And birds are attracted to artificial light at night (though scientists aren’t sure exactly why), so perhaps they’re being drawn in by city lights.
Taking things a step further, Jimenez and his colleagues searched for signs of the so-called “luxury effect,” the tendency of urban wildlife to congregate in high-income neighborhoods due to the greater amounts of green space. Analyzing bird stopover use of more than 2,000 parks across 88 urban areas, they found that stopover density was indeed higher, on average, in areas with higher-income residents.
These nationwide averages, however, don’t tell the full story. Both the overall density of urban stopovers and the strength of the luxury effect varied considerably from one U.S. region to another, and the reason may have something to do with water.
Cities where the luxury effect was most pronounced, such as Phoenix and Los Angeles, were in regions where surface water can be scarce. Dryer regions also had a higher overall proportion of urban stopover sites. It seems in dry places, the way that humans concentrate the available water (and the resulting vegetation) in the places where we live — and especially in the highest-income neighborhoods — may also attract high concentrations of migrating birds.
“This area, where ecology meets the social forces that shape biodiversity, is really important and interesting,” says Emily Cohen, a bird migration expert and faculty member University of Maryland Center for Environmental Science who was not involved in the project. “Not only are cities important for birds, but the connection between people and birds [that can happen in cities] is just a really powerful tool for conservation.”
Cohen says she’d love to see follow-up research on the regional variations uncovered by Jimenez’s work, as well as on how the birds using these urban habitats are actually faring.
“I would describe this paper as more opening up questions than giving answers,” agrees Jimenez. Having completed his Ph.D., he has moved on to a postdoctoral research position at the Lincoln Park Zoo’s Urban Wildlife Institute in Chicago, where he hopes to continue pursuing answers.
But what we definitely know, he says, is that “the actions that we take where we live, which for most people today is in cities — those matter a lot for migratory birds.”
Previously in The Revelator:
What City Birds Around the World Have in Common
