New York City’s annual September 11 Tribute in Light seeks to console a still-damaged city and commemorate those lost in the 2001 Twin Tower attacks. Unfortunately, the stunning beams of light also mesmerize birds, sometimes luring them to their deaths. According to a study published in the Proceedings of the National Academy of Sciences, the light event endangers nearly 160,000 birds each year.

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September is prime time for birds and other animals using the migration corridor that passes through New York City. Everything from warblers to bats to peregrine falcons flutter and swoop above the cityscape, as they have for thousands of years before there was even a single light to fly over.

Related: Bird deaths from skyscrapers reaches into the hundreds of millions

The Tribute in Light disrupts birds’ internal compasses. Birds rely on natural guideposts, such as light from the sun, stars and moon, and the pull of the earth’s magnetic field, to find their way to winter grounds.

“When the installation was illuminated, birds aggregated in high densities, decreased flight speeds, followed circular flight paths and vocalized frequently,” the study authors wrote. “Simulations revealed a high probability of disorientation and subsequent attraction for nearby birds.” This means that while the smaller birds paused, hypnotized by lights, larger birds swooped down and snatched them for supper. Those who elude predators waste precious energy flying in circles over the light show, making them vulnerable to exhaustion and starvation.

“Birds do fly for extended periods of time,” said John Rowden of the National Audubon Society. “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.”

A few scientists and a group of volunteers from the New York City chapter of the Audubon Society are keeping track of the light-dazzled birds. When they count 1,000 trapped birds, the lights are shut off for 20 minutes, allowing the birds to disperse.

Ornithologist Susan Elbin is the director of conservation and science at New York City Audubon Society. As she told the New York Times, “It’s my job to turn the lights out, and I’d rather not have lights on at all, because the artificial light interferes with birds’ natural cues to navigate.”

Via EcoWatch and New York Times

Image via Dennis Leung