Every year, millions of birds migrating at night, often distracted by bright city lights, die by flying into American buildings. Now a study shows where they may be most at risk — and how efforts to save them might be honed.
Chicago, Houston and Dallas are the most dangerous cities for birds traveling at night based on their prime location along one of North America’s busiest migratory routes and the light pollution that they produce, according to the new research.
“The lights will pull those birds in, they’ll circle and they’ll become disoriented,” said Kyle Horton, a postdoctoral researcher at the Cornell Lab of Ornithology and the lead author of the study, published this month in the journal Frontiers in Ecology and the Environment.
Over all, scientists estimate that anywhere from 365 million to nearly a billion birds are killed in such accidents each year in the United States. And while researchers are still investigating the causes, experts believe that many can be attributed to the disorienting allure of artificial light.
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Lights can confuse birds, causing them to fly into one another or a structure. They can also prompt the birds to fly in circles, burning through precious energy reserves stored up for the sometimes long migratory journey.
To better understand that danger, Dr. Horton and his colleagues set out to identify the cities where the greatest numbers of birds encounter the most artificial light during nighttime migration.
“Coming up with rankings was priority No. 1: Where are the greatest threats and how do those threats vary by season?” he said.
To measure light pollution, he and his colleagues collected monthly NASA satellite snapshots over six years. To measure migration, they analyzed activity recorded at nearly 150 weather radar stations over more than two decades.
During spring migration, the cities that pose the greatest threat to birds are Chicago, Houston, Dallas, Los Angeles and St. Louis, they found. (New York ranked eighth.) During fall migration, the most dangerous cities are Chicago, Houston, Dallas, Atlanta and New York.
The researchers also found that, on average, about half of all migrating birds pass through a given city over six nights in the spring and seven nights in the fall. (The nights are not necessarily consecutive.) That, Dr. Horton said, could help direct efforts to save the birds.
“Minimal changes can have a big consequence,” he said. “We don’t expect that cities are going to turn their lights off for 90 straight days, but if they were willing to do it in a dynamic way — they were willing to do it for seven nights — that could be quite impactful.”
It is still not fully understood why birds find artificial lights so disorienting, but the phenomenon has long been documented. Bird deaths have been observed at lighthouses, for example, for more than a century.
About a decade ago, several researchers, including Dr. Horton, found an opportunity to study the phenomenon up close: From 2008 to 2016, they observed birds around the annual Sept. 11 “Tribute in Light Memorial” in Manhattan, in which two bright pillars of light illuminate the night sky.
The results, which they published in 2017, showed that when the lights, which stretch for up to four miles, were on, birds gathered around them in greater densities, flew in circles and vocalized loudly. During brief periods in which the lights were shut off, however, the birds quickly resumed normal behavior.
Since the late 1990s, New York City Audubon has collected reports on dead birds in the region to better understand the problem. From 1997 to 2011, the species with the most fatal accidents with buildings was the white-throated sparrow, the group found.
Generally, smaller birds are more likely to die from flying into buildings, Dr. Horton said. Recent research has also found that songbirds are more susceptible to meeting such ends, suggesting that they may unwittingly lure one another to their death with their lilting calls.
While artificial light has long been identified as a key threat to birds, so, too, has glass. Building glass can be so clear that the birds don’t notice it, or it can reflect nearby trees, tricking birds into flying into it. To solve the problem, some buildings have installed glass with patterns of dots noticeable only up close or glass made with a pattern that reflects ultraviolet light, which is faint or imperceptible to people but not to birds.