Lights Out: The Three Weeks When Cities Decide How Many Birds Arrive
On 9 September, weather radar over Wisconsin counted roughly 103 million birds crossing the state in one night. A week later, nearly 7 million passed over Washington state, with 938,400 of them crossing Clallam County on the Olympic Peninsula alone. Almost all of it happened in the dark, above cities that had their lights on.
Autumn migration in North America runs long, but it has a spine. The Bird Alliance of Oregon puts the peak in the west between 19 September and 19 October, with a second peak in spring from 15 April to 19 May. Audubon Texas marks its own window from 5 September to 29 October. Those calendars come from BirdCast, the Cornell Lab of Ornithology's radar-based forecasting system, which publishes nightly predictions of how many birds will be in the air and where.
Why night flight is the risk
More than 80 percent of North American migratory birds travel at night. The reasons are old and sensible: the air is calmer, the predators are fewer, and the birds navigate by the stars and the earth's magnetic field. Artificial light breaks that system. Skyglow drowns out the stars, city glow pulls disoriented flocks toward built-up areas, and once inside, birds either strike glass or circle a lit building until they burn the energy they need to finish the journey.
The scale is hard to absorb: estimates put building collisions in the United States at 365 million to 988 million birds a year, and at least a million birds a day. A 2024 study added a grimmer correction — of birds taken to wildlife rehabilitation centres after hitting buildings, roughly 60 percent died, which suggests counts of birds found dead on the pavement understate the toll. Light also shifts behaviour: research cited by the Oregon campaign found artificial light can move birds' migration schedules by as much as a week, on top of the direct mortality.
The study that made “lights out” measurable
Chicago has the longest-running evidence in the world, and it started with a hunch. In 1978, Dave Willard, then collections manager at the Field Museum, heard that birds were hitting the McCormick Place convention centre a mile south. He went to look one morning and found four or five dead birds. He kept going, and kept the records.
More than 40 years later, that dataset — 40,000 dead birds, with the window and the date of each — became the backbone of a study in PNAS. Researchers from the Cornell Lab, the University of Michigan, the Field Museum, Colorado State and UMass Amherst compared collisions against lighting records, weather, moonlight and radar counts of how many birds were flying. The strongest factor they could control was illumination. On nights when half the windows were dark, the building recorded 11 times fewer collisions in spring and six times fewer in autumn. Across two decades, halving the lit window area during peak migration would have prevented about 60 percent of the deaths.
“The sheer strength of the link between lighting and collisions was surprising,” said Benjamin Van Doren, then a postdoctoral researcher at the Cornell Lab of Ornithology and the study's lead author. “It speaks to the exciting potential to save birds simply by reducing light pollution.”
The same effect has been seen in reverse, most visibly in New York, where radar tracked birds circling inside the beams of the annual Tribute in Light memorial; when the lights were switched off for 20 minutes, thousands of trapped birds dispersed.
What cities and households can do
The advice is unusually cheap and unusually specific. Turn off unnecessary exterior lighting between 11 p.m. and 6 a.m. through the migration window; close curtains or blinds so interior light does not spill outside; if lights are needed, aim them down and shield them. Chicago's McCormick Place programme is credited with cutting strike rates by around 80 percent at participating buildings.
Glass is the second half of the problem, and it is a daytime hazard: once night lighting has drawn birds into a city, reflective windows remain lethal the next morning. Madison, Wisconsin, became the first city in the state to require bird-safe glass — including visual dot patterns — on new buildings over 10,000 square feet, after volunteers identified a university residence hall as a collision hotspot. Marking that building's glass cut collisions there by roughly 90 percent, and the ordinance survived a legal challenge in 2023. Volunteers in the same programme have now logged more than 5,200 hours surveying 75 buildings.
Listening through a migration night
If you have never heard migration, the species to learn are the small thrushes and warblers that call in flight. Their calls are short and high — a chip or a whistle travelling down through the dark, more often heard after midnight than at dusk. On a big night you can hear dozens in an hour from a quiet garden, which is also why so many are found dead on city pavements the next morning.
Nothing here requires a policy to start. For three weeks a year, the simplest bird conservation available to anyone with a window is a switch.