For millennia, birds have migrated lengthy distances, guided by the moon and stars. But as dark skies dwindle around the globe, scientists wonder how light pollution affects this natural phenomenon. One unlikely light source provided a unique research opportunity.
For one night a year—September 11, in the midst of warblers’ and small passerines’ migration—two beams of light, visible for 60 miles, rise 4 miles into the sky above Manhattan. The “Tribute in Light” installation memorializes the lives lost on 9/11, but it also confuses birds.
Once caught in artificial light, they can circle until they’re exhausted, making them vulnerable to collisions, predators, and dangerous delays in their migration. “It presumably short-circuits birds’ visual cues, and they stay in the lit area,” says Andrew Farnsworth, a research associate at the Cornell Lab of Ornithology.
The tribute’s dramatic contrast between darkness and illumination, coupled with advances in radar technology, inspired Farnsworth and colleagues from the Cornell Lab of Ornithology, University of Oxford, and New York City Audubon. They analyzed radar data from 2008 to 2016, confirming birds would concentrate densely in the beams when the lights were on and return to their migratory paths when the lights were off. Based on that data, scientists persuaded the memorial’s organizers to turn off the lights for 20 minutes whenever 1,000 birds or more were observed in the beams. A heavy migration in 2010 first triggered this response, and the lights were also shut down in four subsequent years.
Farnsworth hopes their work, published in October 2017, will inspire methods for managing other high-intensity light situations. But for now, he feels, the main implication is clear: “We should turn off lights whenever and wherever we can at night, even if only during short periods,” he says. “It makes a difference.”