Each year, billions of migrating birds pass through the United States on their way to seasonal destinations. And each year, these birds are among the hundreds of millions of avian creatures that collide with buildings and die.
As Lindsey Feingold reports for NPR, a new study has identified the metropolitan areas that pose the greatest risk to birds during the spring and fall migrations, with the goal of helping conservationists focus their efforts to protect birds from the man-made structures that stand in their path.
Cities can be death traps for migratory birds for several reasons, including light pollution. Many bird species make their migrations at night, and they are attracted to the glow that emanates from buildings. A recent study found that songbirds that emit chirps known as “flight calls” face a particularly high risk of colliding with illuminated buildings; drawn in by the light, they send out signals that lure other birds to their deaths. Artificial lights also disorient birds, causing them to use their energy flying around and calling out, which in turn makes them more vulnerable to other threats. Yet another issue is the abundance of glass on city buildings, which can reflect birds’ habitat or the sky, leading to collisions. Sometimes, birds will try to fly through buildings’ glass if they can see they sky on the other side. “As a rule,” according to the National Audubon Society, “collisions occur just about anywhere birds and glass coexist.”
But which cities are most dangerous for birds? To find out, a team of researchers looked at more than 20 years of satellite data showing light pollution and weather radar that measures birds’ migration density across the contiguous United States. The results of their investigation, published in the journal Frontiers in Ecology and the Environment, showed that Chicago was the riskiest place for birds in both the fall and spring migratory seasons. Houston and Dallas ranked second and third, respectively, out of lists of ten cities for both seasons.
Because many species do not follow the same routes across different seasons, there were discrepancies between the other rankings for spring and fall. Los Angeles, for instance, was identified as the fourth most dangerous city for birds in spring, but was not included in the list for fall. New York ranked as the fifth most dangerous city for birds in fall, but ranked only eighth in spring. According to the Cornell Lab of Ornithology, this is due to the fact that the West Coast experiences a heavy migration in the spring, while the fall migration tends to be more intense along the Atlantic seaboard.
But Chicago, Houston and Dallas “are uniquely positioned in the heart of North America’s most trafficked aerial corridors,” says Kyle Horton, lead study author and postdoctoral fellow at the Cornell Lab. “This, in combination with being some of the largest cities in the U.S., make them a serious threat to the passage of migrants, regardless of season.”
There are simple actions that can be taken to reduce the frequency of bird collisions—turning lights off at night is chief among them. The Audubon has implemented a national program called “Lights Out,” which seeks to encourage building owners and managers to turn off “excess lighting” during migratory periods. But as the study authors point out, “the intensity and extent of bird migrations vary considerably in space and over time,” so knowing where birds are most at risk during various times of the year can be helpful for implementing targeted conservation efforts.
“Every time new scientific literature comes out, we learn more about the problem,” Kaitlyn Parkins, a conservation biologist at NYC Audubon, tells the Guardian’s Lauren Aratani, “and … we can pinpoint the best solutions using the science.”
Building owners aren’t the only ones who should be cognizant of birds’ migration patterns. According to Horton, around 250,000 birds collide with houses and residences each year, so people living in areas that experience high numbers of migrating birds can do their part to protect the animals.
“If you don’t need lights on, turn them off,” Horton says. "It’s a large-scale issue, but acting even at the very local level to reduce lighting can make a difference.”