The simple fix that could keep us from killing millions of birds

When flocks of migrating songbirds take off from Lake Michigan near Chicago’s Lakeshore Drive, their journeys are often short-lived.

Window lights from the nearby convention center, McCormick Place, along with dense clouds, make it almost certain some of these birds will collide with the building. More than 400,000 birds have died through collisions with the building’s brightly lit windows.

It’s a predictable fate — unless something unpredictable interferes. On Monday, scientists reported that, during Covid-19 lockdowns when the building was vacant, these deaths were curbed.

This finding is part of a study in which researchers analyzed local weather, bird migration, and building lighting patterns in an effort to devise a solution. The answer is as simple as it is effective: turn off the lights.

What’s new — Twenty-one years of data suggest lit windows at Chicago’s McCormick Place significantly increase the likelihood of fatal bird collisions, especially for songbirds.

The researchers published these findings in the journal Proceedings of the National Academy of Sciences.

“Our research provides the best evidence yet that migrating birds are attracted to building lights, often causing them to collide with windows and die,” lead author Benjamin Van Doren, a postdoctoral associate at the Cornell Lab of Ornithology, tells Inverse.

The research found that reducing window lighting by half during the migrating season would:

• Reduce overall bird deaths by 60 percent
• Lead to 11 times lower risk of collisions in spring (6 times lower risk in the fall)

Other factors, such as weather and seasonal patterns and the number of migrating birds, also influenced bird collision rates. Due to migration patterns, Chicago is the city at the greatest risk of light-related bird collisions in the U.S.

Researchers have previously associated high-rise buildings with a higher risk of fatal bird collisions. But the researchers’ findings demonstrate that low-rise buildings like McCormick Place can be a significant threat to birds too — if they’re lit at night.

“For me, the sheer strength of the link between lighting and collisions was surprising—and it speaks to the exciting potential to save birds simply by reducing light pollution,” Van Doren says.

This light pollution exists stems from three long-lasting influences. Historically, office buildings are illuminated at night because of:

• Janitorial services
• Safety purposes
• Security reasons

How they did it — Using data from collisions over a 21-year-period at McCormick Place, along with local bird migration data and weather patterns, researchers created a collision model capable of accurately predicting the risk of bird collisions under different lighting conditions.

“Most birds migrate at night and light pollution can dramatically affect bird migration,” Van Doren says.

Through this method, researchers found lighting at individual windows made a significant difference in whether bird collisions occurred, regardless of overall light pollution. Factors such as wind patterns and the location of lit windows — windows on the north and east sides of the building had higher collision rates — played a significant role.

Birds migrating in spring might also have different risks of colliding with the building due to flying at lower altitudes in spring compared to fall. Of 11,567 total collisions reported over the course of two decades, 64.8 percent occurred during the fall.

The study team also found birds had the highest rates of collision later on or in the “middle third” of the night due to migration patterns. Nearly half of all collisions occurred on high-risk nights when more birds were migrating.

Overall, the model suggests that turning off or reducing light under specific high-risk scenarios — depending on weather conditions and migration patterns — could reduce bird collisions significantly.

Why it matters — Hundreds of millions of birds die each year via what are called “unnatural circumstances,” such as building collisions.

“North America has lost nearly 3 billion birds over the last 50 years, and we need to take action now if we are to halt this decline,” Van Doren says.

A fairly simple solution is already here, the study team argues Cutting the amount of window lighting in half could lead to a nearly 60 percent reduction in bird collisions.

“... we need to take action now if we are to halt this decline.”

The study also demonstrates the consequences of failing to act: If all of McCormick Place’s windows were lit during migration season — the building stopping automatically lighting all of its windows in 1999 — then bird deaths would have increased by a shocking 116 percent.

“Our study demonstrates that turning off lights can substantially reduce these unnecessary deaths,” Van Doren says.

“Birds migrating at night are frequently attracted to and disoriented by lights on the ground, which puts them at risk to collide with lit structures,” he adds. Birds drawn to buildings by bright lights are also more likely to encounter other hazards, such as prowling cats or lack of green space for food, Van Doren explains.

What’s next — The study argues that we need to incorporate “lights-out” initiatives in cities across the country to reduce fatal collisions with migrating birds. Philadelphia passed one such initiative earlier this year.

“‘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.

The study makes two specific recommendations.

First: incorporate weather and migration patterns into these lights-outs programs for the greatest chance of reducing bird collisions.

“I work at the Cornell Lab of Ornithology on the BirdCast project, where we produce migration forecasts to alert the public of upcoming nights of large bird migration,” Van Doren says. “These forecasts can provide advance warning of nights with high collision risk, allowing people to turn off or shield lights.”

Second: reduce — or even better, completely turn off — light sources in building windows at night. Individuals can apply these findings to reduce the likelihood of collisions with their own homes, too.

“Absolutely, turning off lights at certain times is important — we recommend 11 pm to 6 am — but simply using blinds and curtains is also highly effective,” Van Doren says.

He also suggests following bird-safe building models, which incorporate ways to reduce light, when designing new buildings.

“I am excited by the potential to apply our results to make a difference,” Van Doren says.

Abstract: Millions of nocturnally migrating birds die each year from collisions with built structures, especially brightly illuminated buildings and communication towers. Reducing this source of mortality requires knowledge of important behavioral, meteorological, and anthropogenic factors, yet we lack an understanding of the interacting roles of migration, artificial lighting, and weather conditions in causing fatal bird collisions. Using two decades of collision surveys and concurrent weather and migration measures, we model numbers of collisions occurring at a large urban buildingin Chicago. We find that the magnitude of nocturnal bird migration, building light output, and wind conditions are the most important predictors of fatal collisions. The greatest mortalityoccurred when the building was brightly lit during large noctur-nal migration events and when winds concentrated birds along the Chicago lakeshore. We estimate that halving lighted windowarea decreases collision counts by 11×in spring and 6×in fall. Bird mortality could be reduced by∼60% at this site by decreasing lighted window area to minimum levels historically recorded. Our study provides strong support for a relationship between nocturnal migration magnitude and urban bird mortality, mediated by light pollution and local atmospheric conditions. Although our research focuses on a single site, our findings have global implications for reducing or eliminating a critically important cause of bird mortality.