Exactly one year after the Tribute in Light at ground zero drew thousands of migrating birds into its beams, the night of September 11 passed without avian incident.

New York City Audubon volunteers who monitored the lights all night reported a peak of about 100 birds around 2 a.m., and about 700 altogether. It was a far cry from last year, when an estimated 10,000 birds were caught over the course of the night, circling in confusion until the lights were temporarily shuttered five separate times.

That night was almost perfectly designed to lure birds traveling down the Atlantic Flyway, one of North America’s four major conduits for birds migrating to South and Central America. Several nights of storms kept birds grounded in the wetlands north of New York City; on the night of Sept. 11, a favorable tailwind released them, but dense cloudcover hid the stars birds use to navigate and calibrate inborn geomagnetic compasses. The Tribute in Light’s bank of 88 7,000-watt xenon searchlights dominated the sky.

Birds circling in the Tribute in Light on Sept. 11, 2010. Image: John DeGuzman

“Attraction to light is deep in the ancestral instinct, especially in a situation where there’s cloud cover,” said ornithologist Andrew Farnsworth of Cornell University. “A bright light trumps all.” In the birds came.

Though not in direct physical danger from the lights, a night spent circling can be tiring. In previous years, before the New York Audubon Society started monitoring the lights, reports of Manhattanites waking to find exhausted migrants resting on their balconies were not uncommon.

On this Sept. 11, very different conditions prevailed, said Farnsworth. The weather over several previous nights was favorable for flight, so there was no buildup. On the night of 9/11, light winds from the south, blowing against the birds, discouraged flight. “It’s early in the season,” Farnsworth said. “They’re in no tremendous rush.”

The night was partially cloudy, but not so cloudy as to confuse birds, which last year included various species of tanagers, warblers, thrushes and orioles. The 2 a.m. buildup occurred during a moment of cloudiness, but “the moon popped out for a second, and away they went,” said New York City Audubon volunteer Adriana Palmer.

Note: Farnsworth has designed birdcall-recognition software that allows people to build DIY bird trackers and is an organizer of the BirdCast project, in which citizens help scientists track migration. Palmer organizes Lights Out New York, a program that helps city busisnesses and building owners turn off their lights at night to avoid attracting migratory birds. About 90,000 birds die each year in building collisions in New York City. Nationwide, the toll is at least 100 million.

Images & Video: 1) Birds and insects fly in the Tribute in Light on Sept. 11, 2011 (Brandon Keim/Wired.com) 2) Birds circling the Tribute in Light on Sept. 11, 2010 (John DeGuzman/Flickr) 3) Tribute in Light, Sept. 11 2010 (Stayc St. Onge/Vimeo)

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