Every spring and fall, billions of migratory birds slip through the night sky as they shuttle between their winter and summer ranges. Migrating at night helps them avoid predators and overheating, while also freeing them up to eat during the day. They use stars for orientation, but some also tweet while flying, emitting subtle flight calls that help with navigation and other group decisions.
When they fly through urban areas at night, migrating birds are often confused by electric lights, which can disorient them and lure them to crash. A glowing high-rise can kill hundreds of migrating songbirds in a single night, a problem that has begun to draw more public attention in recent years. In U.S. cities like New York, Chicago and Houston, some skyscrapers and other landmarks now institute "lights out" programs during bird migration seasons.
This has helped, but as researchers highlight in a new study, light pollution remains a major problem for migratory birds. Not only do large numbers still fall victim to brightly lit buildings, the study found, but species that produce flight calls seem to be more vulnerable than their quieter counterparts.
Previous research has shown that birds produce more flight calls over bright cities than over darker rural areas, suggesting light pollution changes their behavior by prompting them to communicate more while flying. And in the new study, published in the Proceedings of the Royal Society B, researchers found that illuminated buildings take a much higher toll on night-calling species.
"Nocturnal flight calls likely evolved to facilitate collective decision-making among birds during navigation," says study co-author Benjamin Winger, an evolutionary biologist at the University of Michigan, in a statement. Unfortunately, he adds, "this same social behavior may now exacerbate vulnerability to a widespread anthropogenic disturbance: artificial light from buildings."
To test that idea, Winger and his colleagues examined sets of bird-collision data from Chicago and Cleveland, two cities located on a major north-south flyway for migrating birds. The Chicago dataset features nearly 70,000 collisions dating back to 1978, while the Cleveland dataset is smaller, having begun in 2017. Of the 93 bird species in these records, a few flight-calling sparrows, thrushes and warblers represent the bulk of fatal collisions, the study showed, accounting for thousands of deaths. The five that appear in the records most often are white-throated sparrows, dark-eyed juncos, song sparrows, swamp sparrows and ovenbirds.
When the researchers compared all the birds' collision rates with population sizes, these "super collider" species turned out to be overrepresented, while birds that don't produce flight calls were underrepresented.
Since flight calls seem to help migratory birds make collective decisions in the darkness, the researchers explain, individuals might vocally beckon one another when they become confused by artificial light. "This relationship may spawn a vicious cycle of increased mortality rates if disoriented individuals lead other migrating individuals to sources of artificial light," they write.
Chicago can be a particularly dangerous place for migratory birds, and as another recent study found, its illuminated buildings collectively expose migrating birds to more artificial light than any other U.S. city. In the new study, researchers discovered that when more lights were left on at Chicago's McCormick Place convention center — a notorious hazard for migrating birds — more night-calling birds fatally collided with the convention center. For species that don't make flight calls, however, the amount of light from the convention center had no significant effect on collision rates.
While this correlation may not prove that more artificial light causes more deaths of night-calling species, it does make a strong case for further research into that possibility. And since it's known that light pollution threatens migratory birds in general, this points to a relatively simple solution: turning off more outdoor lights at night.
According to study co-author David Willard, a retired ornithologist at Chicago's Field Museum, while McCormick Place "remains one of the most dangerous buildings in Chicago for night-migrating birds," it has already reduced bird collisions by 75 percent since 1978 by adjusting its illumination. "Our new analysis shows that implementing further reductions in light here and elsewhere in Chicago will help greatly to reduce bird fatalities," Willard says.
And even if most of us aren't in a position to save as many birds as the managers of skyscrapers, stadiums and convention centers are, we may not be powerless to play a role. As University of Windsor ornithologist Dan Mennill points out in The Conversation, "the impact of artificial lights can be mitigated by an easy change to our own behavior: the flip of a light switch."