By Mary Beth Nevulis, Louisville Audubon Vice President and Lights Out Louisville Program Chair
For migrating birds, there’s a dark side to light. In fact, it can be downright fatal.
Not all light, of course. For the roughly 70% of North American bird species that migrate at night, celestial objects that emanate natural light are key for navigation. German ornithologist Franz Sauer proved this in the 1950s by placing migratory birds inside a planetarium with a movable artificial sky: As he and his researchers rotated the ersatz sky, the birds would reorient their flight position to align with certain constellations—including the North Star—during seasonal migration periods.
But when excessive artificial light at night washes out those celestial cues, birds lose one of their most important wayfinders, and can end up veering from their normal flight paths.
Worse still: Artificial light acts like a beacon for birds, luring them toward buildings and homes. Some become so disoriented they end up circling brightly lit manmade structures until they’re exhausted, eventually landing on the ground—where their fatigue makes them easy pickings for predators—or simply dropping dead from the sky.
And—too often—the effects of artificial light at night result in a window or building collision. Sometimes the result is immediately fatal. Other times, birds that strike a building fly off and seem to survive. But research published in 2024 by scientists at Fordham University, NYC Bird Alliance, American Bird Conservancy, and Stony Brook University found 60% of collision victims ultimately died from their injuries—even birds treated by wildlife rehabilitators.
Artificial light doesn’t just kill birds outright; it threatens entire populations. Birds delayed by nighttime disorientation may arrive late to breeding grounds, miss critical food sources like caterpillar hatches, or fail to raise healthy young. Over time, these disruptions ultimately impact species survival.
The staggering scale of the problem
Bird collisions happen every day, especially during migration seasons, and they add up to a mind-boggling number of deaths. In the U.S., research shows more than a billion birds—possibly as many as 5 billion birds—die every year after colliding with buildings.
While skyscrapers often get the blame, most bird deaths actually happen closer to home—all of our homes. In fact, one- to three-story buildings—which describes most residential homes—account for almost half (44%) of annual bird-collision deaths in the U.S.
To understand how estimates of annual bird-collision deaths reach into the billions, consider this: Kentucky has almost 2 million occupied housing units, per 2024 United States Census Bureau data. The math isn’t hard to do from there: A single fatal collision each year at every home in Kentucky would total 2 million dead birds a year—just in our state. So each individual strike may seem small, but across cities, counties, and the country, the bodies pile up quickly.
But if you look at the math from another angle, it has the potential to be a boon for bird conservation: If every home in Kentucky prevented just one fatal collision a year, we’d collectively save 2 million birds every year, and there are plenty of easy, fast, low-or no-cost ways to reduce artificial light at night.
Small changes, big impact
Whether for a skyscraper or a single-story home, simple actions really can make a difference—especially during peak migration months (April and May in the spring; September and October in the fall) and times (11 p.m.–6 a.m.), when millions of birds are traveling the major migratory flyway over Kentucky.
The Louisville Audubon Society’s Lights Out Louisville program—a conservation initiative that asks residents, businesses, and building operators in Kentucky and Southern Indiana to reduce artificial light at night to protect migrating birds—recommends:
- Turn off lights when not in use
- Draw window coverings after dark
- Turn off decorative outdoor lighting, or limit it to ground level
- Use timers or motion sensors for essential outdoor lights
- Add shields to direct outdoor lighting downward
- Choose warmer-toned bulbs (below 2700 Kelvin) for outdoor lights
Reducing artificial light at night also brings benefits beyond bird conservation, of course: lower energy bills (and therefore cost savings), reduced carbon emissions, and darker skies for stargazing.
Doing something good for the world doesn’t have to be hard or expensive or burdensome. Taking a few simple steps to reduce unnecessary light at night, even for just part of the year, will save birds—maybe even millions of birds. Anything we can do matters—because everything adds up.
Featured photo: Purple Martin, Keith Kingdon/Audubon Photography Awards
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