Scientists have long known that artificial light can attract and disorient birds at night, causing collisions and wreaking mischief with their migratory path. Now, the annual September 11 “Tribute in Light Memorial” in Manhattan has provided a unique opportunity to study and quantify the effect.

The multiyear study, published in the Proceedings of the National Academy of Sciences, found that birds gathered in greater densities, flew repeatedly in circles and vocalized loudly when the memorial’s powerful beams were illuminated.

However, when the lights were turned off for brief periods, the birds were quick to resume their normal flight paths and behaviors. Although the researchers were not calling for any changes to the annual event, their findings suggest a simple fix for ongoing light pollution in other places.

“Wherever we can turn lights off at night, we should be doing it,” said Andrew Farnsworth, an ornithologist with Cornell University and an author of the study, which claims to be the first to quantify bird responses to urban nighttime light.

Researchers from Cornell and the New York City Audubon Society have been monitoring the memorial, which consists of two pillars of 44 spotlights aimed directly upward to simulate the fallen twin towers, since it was first presented in 2002. In 2008, the team began using radar and acoustic sensors to track how many birds the light was attracting and how it affected their behavior.

In 2010, the beams attracted so many birds that the researchers convinced the memorial’s operators to turn off the lights for 20 minute intervals, which presented “a unique opportunity” to study “behavior in birds when these incredibly powerful lights were on versus when they were off,” said Farnsworth. The lights were briefly extinguished again in 2012, 2013, 2015 and 2016.

Compiling data from seven nonconsecutive years, the researchers found that bird density near the instillation was 20 times greater than surrounding areas, causing sometimes fatal collisions with structures and other birds. Alterations to the birds’ migratory paths also put them at risk of death and starvation from arriving late to their destinations.

All such behaviors ceased within minutes of the lights being turned off. The installation affected more than 1.1 million birds cumulatively in those seven years, the researchers said.

Though the study makes no specific recommendations about the 9/11 Memorial, it does recommend that cities consider “selective removal of light during nights with substantial bird migration.” Sports stadiums, car dealerships, mountaintop monuments and large buildings are among the worst offenders when it comes to nighttime light, said Farnsworth.

© 2017 The New York Times Company