The Twin Cities has been named one of the worst urban areas in the country for migrating birds, a result of its bright nighttime lights and location in a flyway with a huge volume of birds pouring through.
Chicago, Houston and Dallas topped the list.
Minneapolis sits at the top of the Mississippi Flyway, the primary navigation corridor for an estimated 60 percent of North American migrating bird species. That helps explain why it finished near the top of 125 cities analyzed in a new study by the Cornell Lab of Ornithology.
The Twin Cities ranked No. 6 in the spring migration and No 7 in the fall. Boston, by contrast, is not near the flyway and ranked 36 and 24.
Although researchers have written extensively about light pollution and disoriented birds crashing into buildings, the Cornell research breaks new ground, the authors say, by taking light radiance data from satellites and overlaying it with bird counts from weather surveillance radar.
The data on migrating birds from more than 140 weather stations was the new element, said Kyle Horton, a Cornell researcher and lead author of the study, which will be published in the May issue of Frontiers in Ecology and the Environment.
"It's probably one of the only tools for detecting the broad passage of birds," Horton said. "These are some of the first U.S.-wide depictions of migration that we've had access to."
Climate change and cats still pose greater threats to rapidly declining bird populations, Horton said, but light pollution poses a notable threat.
That's because most birds migrate, and most favor nighttime. They tend to be safer from predators in the dark, for example, and weather tends to be cooler and calmer.
As a result, birds often use the stars and polarized light from the setting sun to set their compasses and navigate. But night flight leaves them vulnerable to light pollution from streetlights, office buildings, homes, car dealerships, stadiums and other buildings.
Intense glow at night attracts birds, who sometimes fixate on lights, much like moths on a porch light, Horton said. Some birds become entranced — circling and circling, sometimes all night. In extreme cases, the birds grow exhausted and collide with buildings. The risk of mortality for young birds probably increases in the fall because they aren't experienced fliers.
Scientists estimate that about 600 million birds die each year in the U.S. from striking buildings, communications towers, wind turbines and other structures, although not always because of artificial light.
In other cases, the researchers said, birds survive and escape the disorienting light but burn a lot of energy that they need to reach their remote destinations, potentially reducing their ability to breed. Scientists haven't figured out how to measure the impact of this diffuse energy expenditure, Horton said.
Which birds are most affected by light pollution isn't clear. Warblers seem particularly attracted to lights, Horton said, but researchers don't know why.
Common city dwellers such as pigeons and crows aren't the issue, according to Ashley Peters, a spokeswoman for Audubon Minnesota, which has worked on bird collisions for years. Peters said it's other birds such as songbirds, hummingbirds, indigo buntings, woodpeckers and owls. She cited a YouTube video showing birds trapped in light columns beamed into the sky as part of a 9/11 tribute in New York City.
"It's probably the most dramatic filmed version that I've seen," Peters said.
Horton said he hopes the Cornell findings will help conservation groups target their work on cities where it's most needed.
Like the Twin Cities.
Lights Out campaign
About 70 buildings around the region now participate in Audubon Minnesota's voluntary Lights Out campaign, including some in Rochester and many state government buildings in St. Paul. Participants agree to turn down or switch off lights between midnight and dawn for six weeks, two times a year, during peak spring and fall migration seasons.
Peters, too, cited the importance of the Mississippi River. The river flyway guides many birds and "provides a consistent stretch for habitat and areas they can fish and find protection," she said.
Kevin Lewis, executive director of the Building Owners and Managers Association of Greater Minneapolis, said most of his downtown members participate in the Lights Out project.
But compliance isn't always simple. Scott Lambert, president of the Minnesota Automobile Dealers Association, said dealers keep the lights on at night to thwart thieves. He said he doubts they'll change the practice.
"We've got a well-documented problem with after-dark, nighttime thefts at auto dealers," Lambert said. "They're after the rims and tires."
Peters said Audubon Minnesota is working to add U.S. Bank Stadium to the Lights Out list. The Vikings and the Minnesota Sports Facilities Authority have committed to joining the project, she said, but the parties are "still discussing the logistics of that."
Audubon is wrapping up its own survey of bird collisions in downtown Minneapolis, including those at the new stadium, to be released later this year.
Peters agreed with Horton that light pollution is just one of several threats to birds. But it's one that's relatively easy to address, she said.
"It's something that people can understand. It's something within most building managers' control."
Jennifer Bjorhus • 612-673-4683
