On the evening of the ninth anniversary of 9/11, the twin columns of light projected as a memorial over the World Trade Center site became a source of mystery.

Illuminated in the beams were thousands of small white objects, sparkling and spiraling, unlike anything seen on other nights. Some viewers wondered if they were scraps of paper or plastic caught in updrafts from the spotlights' heat. From beneath, it was at times like gazing into a snowstorm. It was hard not to think of souls.

Those unidentified objects have now been identified as birds, pulled from their migratory path and bedazzled by the light in a perfect, poignant storm of avian disorientation.

"It's only happened once before. It's a confluence of circumstances that come together to cause this," said John Rowden, citizen science director at the Audubon Society's New York City chapter. "Some of it has to do with meteorological conditions, and some with the phase of the moon."

New York City sits in the middle of a major migration corridor, used for millennia by birds flying south for the winter. During autumn nights, thousands of birds pass directly above the megalopolis, a passage generally unnoticed by its human inhabitants.

During the previous week, weather was bad for migration. Tropical storm systems moved north up the U.S. East Coast, pushing against birds headed south. To conserve energy, migratory birds prefer tailwinds, and are willing to wait for good weather.

"Birds were coming down from the north and piling up, waiting to push southwards," said Rowden.

To navigate, birds rely on a variety of internal compass mechanisms, which are calibrated to Earth's geomagnetic fields by sunlight, starlight and moonlight. On Sept. 11, the new moon was just two nights old, a thumbnail sliver. In such conditions, birds rely on starlight, but parts of the lower Manhattan sky were overcast.

The buildings resembled stars. Outshining them all was the Tribute in Light above Ground Zero.

Rowden estimates that 10,000 birds entered the beams, becoming confused and circling until the Municipal Art Society, working with New York City Audubon, shut the lights for 20 minutes, allowing the birds to leave. That happened five times over the course of the night.

The spotlights were not directly dangerous to the birds. Instead, risk comes from wasted time and energy needed for later.

"Birds do fly for extended periods of time. It's not that they can't do it. But they're doing it to get south of here. If they spend all their time in that small area, they won't get to good foraging habitat, and it will compromise them for later parts of their migration," Rowden said. "But I feel that we did allow them to get out."

Volunteers from New York Audubon identified American Redstarts and Yellow Warblers. Wood Thrushes, Bicknell's Thrushes, Baltimore Orioles and various species of Tanager may also have been trapped. Recordings of flight calls inside the light columns are now being analyzed at the Cornell Lab of Ornithology.

According to Rowden, the only previous comparable event was in 2004. Last year, barely a dozen birds were trapped by the lights. The problem, however, is not unique to the 9/11 memorial, but posed by tall, brightly lit buildings in most major cities.

To limit the toll, New York Audubon organized the Lights Out New York program, for which many prominent commercial structures – including the Chrysler Building and Rockefeller Center – turn off or mask their lights during the migration season.

Just before 5 a.m. on Sept. 12, the lights were turned off and on for the final time, said Rowden. In the next hour, birds gathered again.

"As soon as they could get any visual horizon, they could use that as a cue and navigate their way out," he said. With dawn the birds departed.

Images: 1) The Tribute in Light around 9pm on September 11./Brandon Keim. 2. Birds in the lights./Flickr, Kai Schreiber. 3) Video from lower Manhattan on September 11; footage of birds begins at 0:35./Daniel Turkewitz.

See Also:

Brandon Keim's Twitter stream and reportorial outtakes; Wired Science on Twitter. Brandon is currently working on an ecological tipping point project.