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Two blue beams of light will shine upward from lower Manhattan tonight, an annual “Tribute in Light” to commemorate the September 11 terrorism attacks. Standing right beneath the beams will be New York City ̽����ѡ volunteers—on hand to save the birds trapped in the lights’ ambit.

For reasons still unknown to science, artificial light attracts birds, from fledgling seabirds to (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 ̽����ѡ 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 need for such monitoring was almost immediately apparent after the tradition started in 2002. Some initial skepticism aside, those behind the light tribute have��been wholly receptive to New York City ̽����ѡ’s suggestion, and over the past decade-plus of monitoring, several 9/11 survivors have volunteered.

“[Survivors] have told me the last thing they want from this memorial, which is so meaningful and beautiful…is for there to be more death on this spot,” says Debra Kriensky, a conservation biologist at New York City ̽����ѡ who helps organize the monitoring.

Flying high in the air, caught birds are just distant specks, easily confused for confetti, says Susan Elbin, director of conservation and science at New York City ̽����ѡ. In an attempt to better identify species, volunteers record the birds’ flight vocalizations—American Redstarts, Ovenbirds, Black-and-white Warblers and Northern Parulas have been especially prevalent.

While birds are scarce in some years, this year is expected to be plentiful, thanks to, among other things, the new moon. In 2010, a new moon and cloudy skies yielded an estimated 10,000 birds caught through the night—the lights had to be shut down five times.

Tonight, if—or more likely, when—the lights go off, remember it’s just to free the birds. ��

Correction:��The��Municipal Art Society of New York handed off operation of the tribute lights to the National September 11 Memorial and Museum. This story has been updated to reflect that.