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Study at September 11th tribute site confirms dramatic impact of light on migrating birds

This photo, taken September 11, 2017, looks up from inside an array of lights in New York City. Most, if not all, of the specks in the photo are birds. Photo by Kyle Horton

For more than a century, people have known that artificial light is a detriment to birds migrating during darkness. The morning after a foggy or stormy night, birds are often found dead on the ground around lit buildings, lighthouses, or other structures.

What remains poorly known is how exactly light influences the behaviors of actively migrating birds. Today, researchers with the Cornell Lab of Ornithology, the University of Oxford, and New York City Audubon have published a study that provides some of the most compelling evidence yet that artificial light at night causes radical changes in the behaviors of migrating birds.

The study was conducted at a site that is steeped in our national consciousness: six blocks south of the World Trade Center in New York City, where the Tribute in Light memorial is held each year to commemorate the September 11, 2001, terrorist attacks. Two beams of light — each with 44 xenon bulbs of 7,000 watts — rise into the night sky, mirroring the twin towers of the World Trade Center where nearly 3,000 lives were lost. The display, which occurs only on September 11 each year, is visible from a 60-mile radius around lower Manhattan.

Well before the study at the installation began, New York City Audubon reached out to the original tribute organizers, the Municipal Art Society, to let them know about the impacts of artificial light on migratory birds. In 2002, the two organizations worked together to develop a protocol to save the affected birds. When more than 1,000 birds are seen circling in the beams or flying dangerously low with frequent calling, the tribute lights are turned off for approximately 20 minutes. The National September 11 Memorial & Museum, which took over as tribute organizers in 2012, has continued the practice.

The brief interludes of darkness during the nightlong tribute provided a unique opportunity for the scientists to quantify changes in bird behavior in several ways during the alternating periods of light and darkness. (A 2015 documentary film about threats to songbirds, The Messenger, featured the tribute display and included an interview with the Cornell Lab’s Andrew Farnsworth, a co-author of the new study.)

‘Very difficult for migrating birds to avoid’

The scientists, whose work was published in the Proceedings of the National Academy of Sciences, wanted to know if poor visibility conditions are necessary for birds to be affected and how far outside the illuminated area the impacts of light extend. They conducted the study the night of the tribute on seven of the last nine years and estimate the installation influenced the migration behavior of about 1.1 million birds and at altitudes up to 2.15 miles (4 km).

“Our study combines human observations with data from radar and acoustic sensors to more completely reveal the extent to which bright lights can impact actively migrating birds,” says lead author Benjamin Van Doren, a doctoral student at Oxford’s Edward Grey Institute. “We learned that a single bright light source can attract congregations of tens of thousands of migrating birds and that effects are felt by birds kilometers away. Moreover, we learned that light can strongly affect birds even when it isn’t foggy or stormy.

“Our simulations suggest that it is very difficult for birds flying close to the lights to avoid being attracted. Perhaps most importantly, birds rapidly returned to normal behavior when the lights were temporarily shut down. This is encouraging, as it suggests that strategic and sparing shutdowns of disruptive lights, for example during nights when large numbers of birds are migrating, can have a large positive impact.”

A man walks among the lights as they aim skyward. Photo by Ben Norman

The paper notes that in 2015 and 2016, observers reported that “many birds collided with the glass windows of a building under construction just north of the lights.” The extent of the mortality is unknown, but even for birds that survive their encounter with the bright lights, the effects are less than ideal.

“They slow down, start circling, and call more frequently,” says Van Doren. “They end up burning energy without making any progress and risk colliding with nearby buildings or being caught by predators.”

When the tribute was illuminated, the study’s authors found that densities of birds over lower Manhattan could reach 60 to 150 times the number that would typically be found in the area at that time. The impact on birds was consistent even on clear nights. Many previous studies focused on the dangers posed by artificial light on nights with poor visibility.

“We had more than 20 wonderful volunteers on the ground counting birds to confirm what was happening in the beams,” says Farnsworth. “We also used remote sensing tools, including data from local National Weather Service radars, to understand the density and movements of the birds. Acoustic monitors recorded call notes and captured the vocal behavior of birds in the beams. And we ran computer simulations to try to better understand the dynamics of the patterns we were observing.”

The researchers stress that they aren’t suggesting the tribute should be brought to an end.

“We deeply respect the intent and spirit of the tribute to the lives lost during the September 11th attacks, and our initial hope was to work with the organizers to reduce potentially hazardous situations for birds,” says Van Doren. “It is encouraging for conservation that the organizers of such a public and evocative display were willing to take actions to reduce its effect on birds. We hope that others will follow this example of how the negative impacts of lights on birds can be mitigated with strategic and sparing use of the ‘off’ switch.”

Similar light installations in which high-intensity beams are pointed vertically are rare, but horizontal and angled lights are used at all sorts of locations. Impacts on birds, including mortality, have been documented at such sites time and time again.

“We recommend building lights be turned off for as much of the night as possible, but at least from midnight to dawn during migration season,” says study co-author Susan Elbin of New York City Audubon. “This is true for areas around homes as well as other brightly lit areas such as sports stadiums, construction sites, offshore oil rigs, and large buildings. Migrating is already hard enough for birds without this added danger from artificial light at night.”

Van Doren adds: “As we continue to improve our ability to forecast birds’ migratory movements (see, we hope that targeted, short-term shutdowns of lights of all kinds will become common. — Matt Mendenhall, Editor

Read the paper

Benjamin Van Doren, Kyle Horton, Adriaan Dokter, Holger Klinck, Susan Elbin, Andrew Farnsworth. High-intensity urban light installation dramatically alters nocturnal bird migration. Proceedings of the National Academy of Sciences. September 2017. Abstract.

Read more news about bird migration

Our review of The Messenger

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