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 Dr. 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 installation 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.