Wisconsin skies filled with an estimated 103.3 million migrating birds overnight on September 9. Hundreds of millions more are expected to pass through the state over the coming weeks as fall migration reaches its height, and wildlife officials say the surge brings a familiar danger: lit-up windows that can prove fatal to disoriented, exhausted birds.
The overnight numbers come from BirdCast, which uses NEXRAD weather radar across the state to estimate how many birds are moving and where. The Wisconsin Department of Natural Resources flagged the black-throated green warbler as one example of the small songbirds now crossing the state. Most migrating birds travel at night, and artificial light can draw them into urban areas, where they face a much higher risk of striking windows.
Why Nighttime Light Is the Real Hazard
The DNR's warning centers on a simple mechanism: artificial light disorients migrating birds and pulls them toward cities, where glass becomes a deadly obstacle. Window collisions are described as one of the leading threats to bird populations, killing between 365 million and more than 1 billion birds annually in the United States, according to the U.S. Fish and Wildlife Service. As part of the Lights Out Wisconsin campaign, residents are encouraged to turn off non-essential outdoor lighting between 11 p.m. and 6 a.m. during the critical migration window.
The lethality of these collisions may be undercounted. A 2024 study published in PLOS ONE found that 60% of birds brought to wildlife rehabilitation centers after hitting buildings ultimately died, according to Hoodline's earlier reporting, suggesting that mortality estimates based only on birds found dead at the scene understate the true toll.
Chicago's Data Point for Wisconsin's Skyline
Chicago has also grappled with bird collisions, Hoodline previously reported. The example is relevant to Wisconsin cities because reducing unnecessary light can help make urban areas safer for migrating birds.
The DNR's advice for homeowners is straightforward: making windows more visible can help reduce collisions. Those looking to track the scale of nightly movement can sign up for Cornell Lab of Ornithology BirdCast migration alerts, which notify subscribers when major bird movements are expected in their region.
Madison's Building Code Sets a Precedent
Structural fixes have already taken hold in Madison, which in 2020 became the first Wisconsin municipality to require bird-safe glass, including visual dot patterns, on new construction over 10,000 square feet. The ordinance survived a legal challenge from trade associations when the Wisconsin Court of Appeals upheld it in October 2023, according to SOS Save Our Songbirds.
That policy traces back to a 2020 pilot retrofit at UW-Madison's Ogg Residence Hall, which had been flagged by volunteer survey teams as a high-collision hotspot before visual dot markers were applied to its glass. The retrofit produced a 90% reduction in bird collisions, per the UW-Madison Office of Sustainability.
Volunteers continue to track the problem on the ground. The Southern Wisconsin Bird Alliance's Bird Collision Corps has logged more than 5,200 hours surveying 75 buildings, working with commercial property owners, university campuses, and municipal buildings to recommend mitigation strategies across the state.
A Species Already Under Pressure
The black-throated green warbler, the species highlighted in the DNR's alert, is one of the small songbirds now crossing Wisconsin, according to the National Audubon Society. Many migrating songbirds travel at night, making artificial light a concern during the journey.
That reliance on a clear, dark sky is part of why light pollution is such a threat. The broader picture also includes concerns about bird populations. Wisconsin's western border also underscores the importance of stopover habitat for migratory birds.
Wisconsin municipalities are working to expand habitat, reduce urban threats, and educate the public on migratory birds. As hundreds of millions more birds move through the state in the coming weeks, officials are counting on that same mix of household habits and municipal policy to keep the death toll from climbing further.
