New York City’s beautiful 9/11 memorial, the Tribute in Light, had to be turned off and on multiple times last night to make sure migratory birds in the area didn’t lose their way, Gothamist reports.

The tower-like lights that stretch into the air have been a New York 9/11 tradition since 2002. The problem it created for birds, which are attracted and disoriented by lights, became apparent quickly, according to the Audobon Society. Each year, it stations monitors near the installation to make sure that birds aren’t harmed by it.

Volunteers, including members of the New York City Audobon Society and 9/11 survivors, stand by watching the beams in two-hour shifts. If a bird drops to the ground or they see a thousand birds circling in the air, they contact the Municipal Art Society of New York (the organization running the Tribute), which then shuts off the lights for 20 minutes so that the birds can clear out. During last night’s memorial, the lights had to be turned off several times.

A video on YouTube shows what it looks like when thousands of birds flock to a light and then get very, very confused.

While light pollution is a known bird killer, the Society explains on its website that this particular form is extra dangerous because of its enormous size. The Tribute in Light goes four miles into the sky, making it extremely dramatic—and extremely captivating for birds nearby.