How Turning off Half a Building's Lights Saves Birds

It can result in 11 times fewer bird and window collisions.

Chicago skyline view from 360 Chicago observation deck, John Hancock Building
Light at night confuses migrating birds. Yannick Tylle / Getty Images

Turning off just half the lights in a big building at night can result in up to 11 times fewer bird collisions, a new study finds.

Most birds migrate at night using lights from the night sky. But light pollution from buildings attracts and confuses many birds, causing them to fly toward the light. Each year, as many as 1 billion birds are killed in the United States from collisions with buildings and glass windows.

Building on earlier studies, new research finds that even shutting down half the lights can dramatically decrease the number of bird crashes.

Researchers started with more than 40 years of data collected by David Willard from Chicago’s Field Museum. Much of the information was gathered from birds at McCormick Place, North America’s largest convention center which is just a mile south of the museum.

Years ago, Willard began noticing a pattern. On nights when fewer lights were on at McCormick Place because of construction work or holidays, there were fewer dead birds on the ground the next morning. He began collecting data on light patterns, as well as gathering the birds he found on the sidewalk. He quickly saw there was a link between the number of lights and the number of collisions.

In the new study, researchers added more sophistication to that earlier work.

“Our study combined records collected by David Willard and other Field Museum scientists with information on weather conditions and the number of migrating birds flying over Chicago each night,” Benjamin Van Doren, a postdoctoral associate at the Cornell Lab of Ornithology and the paper's first author, tells Treehugger.

“By joining these different sources of data, we were able to understand how lights, weather, and migration each contributes to collision mortality," Van Doren adds. "We developed a statistical model that isolated the impact of lighting while accounting for these other factors.”

The team used Doppler radar to measure the number of birds that were migrating over the city each night. They also used information about weather conditions from a local airport.

“There was a much higher risk for collisions when the radar measured more birds migrating over Chicago,” Van Doren says. “Certain wind conditions also increased risk—specifically, winds blowing from the west, which likely concentrates birds in the airspace along the lakeshore, above Chicago.”

In the new study, researchers discovered a dramatic drop in bird collisions when windows were darkened at McCormick Place. In spring, when half the windows were lit, crashes decreased by 11 times. In fall, collisions dropped by six times when half the windows were darkened.

The results were published in the journal Proceedings of the National Academy of Sciences.

Community Campaigns Make a Difference

Field Museum ornithologist David Willard measured all of the migratory birds analyzed in the study.
Field Museum ornithologist David Willard measured all of the migratory birds analyzed in the study. Field Museum, John Weinstein

The McCormick Center played a key role in this study because it’s a large building on the lakefront with large windows with lots of light output, says Van Doren. “However, McCormick Place is only one example of a wider problem of light pollution,” he says.

Many cities have joined Lights Out campaigns, which voluntarily urge property managers and homeowners to turn off unnecessary external and internal lights at night to protect birds during migration season.

The National Audubon Society created the first Lights Out program in 1999 in Chicago. Now there are nearly three dozen cities with Lights Out programs including Atlanta, Baltimore, Boston, New York, Philadelphia, and Washington, D.C.

The researchers hope these new study findings will encourage people to turn out the lights.

“I am excited by the potential to apply our results to make a difference. “Lights Out” programs and campaigns are gaining momentum in North America—these initiatives encourage buildings and the public to turn out unnecessary lights to save birds,” Van Doren says.

“There is also now a LEED building credit specifically focused on bird-safe building design, which includes bird-safe glass and light reduction among its criteria. Turning off lights at certain times is important (we recommend 11 pm to 6 am), but simply using blinds and curtains is also effective.” 

View Article Sources
  1. Van Doren, Benjamin M., et al. "Drivers of Fatal Bird Collisions in an Urban Center." Proceedings of the National Academy of Sciences, vol. 118, no. 24, 2021, p. e2101666118, doi:10.1073/pnas.2101666118

  2. Loss, Scott R., et al. "Bird–building collisions in the United States: Estimates of annual mortality and species vulnerability." The Condor, 2014.

  3. "Philadelphia Joins National Lights Out Program to Save Migrating Birds." The Academy of Natural Sciences, 2021.