The annual return of the Tribute in Light to mark the September 11th attacks' anniversary also means that birds flying above and around New York City get extremely confused. Which is why the NYC Audubon was on scene to monitor migrating birds last night, and asked the Tribute in Light's organizers to turn off the lights a few times.

The light installation involves "eighty-eight 7,000-watt xenon light bulbs positioned into two 48-foot squares, echoing the shape and orientation of the Twin Towers," according to the 9/11 Memorial. On a clear night, the lights can be seen 60 miles away and extend four miles into the sky.

The Audobon Society has explained what happens:

For reasons still unknown to science, artificial light attracts birds, from fledgling seabirds to migrating songbirds (it does the same to moths). Once captivated, disoriented birds may crash into windows, or spend hours circling.

The 9/11 tribute is particularly problematic: dozens of 7,000-watt bulbs allow it to reach four miles into the sky—it’s visible from 60 miles away. So New York City Audubon members and volunteers take two-hour shifts underneath, scanning the beams, counting birds. Every time 1,000 birds or more are circling—or an exhausted bird falls to the ground—they alert the National September 11 Memorial and Museum (who runs the tribute), which immediately turns off the lights for 20 minutes, giving birds a chance to clear the area.

The NYC Audubon Society added, "We found a lot of warblers early on. American redstart very prevalent. insects are attractive for these birds."

In 2015, the lights were turned off eight times due to a heavy migration. The NYC Audubon is continuing to research the phenomenon.

Correction: This article updated to reflect that the Tribute in Light was turned off three times.