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Migratory birds (up to 10,000) had become trapped in the light beams

...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.
“As soon as they could get any visual horizon, they could use that as a cue and navigate their way out,”
My take: birds spend a lot of time getting into shape for migration - for some species, eating enough and getting their feathers into condition is a fine balance between success and failure. How much energy do the birds consume when they are trapped in the light beams? They may fly off when the lights are switched off, but do they have enough energy for the journey?
"Volunteers from New York Audubon identified American Redstarts and Yellow Warblers. Wood Thrushes, Bicknell’s Thrushes, Baltimore Orioles and various species of Tanager..."
These are not large birds, so the toll on their energy reserves and the margin for error in their flight calculations may be slight.
Amplify’d from www.wired.com
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.
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.
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.
Read more at www.wired.com

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