Glass and Flight Paths: Why Chicago Is Ground Zero for Bird Collisions

On the morning of October 5, 2023, staff from Chicago's Field Museum walked the grounds of McCormick Place and found a scene that shocked even veteran ornithologists. During the night, 954 migrating birds had died after hitting the convention center's Lakeside Center. Most were warblers, small songbirds weighing less than half an ounce, on their way from Canadian forests to Central and South America. It was the deadliest single-building collision event ever recorded. It was also not an accident in any meaningful sense. It was the predictable result of putting a glass wall in the path of one of North America's busiest migration routes.

Chicago's geography makes it especially dangerous. Lake Michigan acts like a funnel. Migrating birds avoid crossing open water, so they follow the shoreline, and the city sits right at the bottom of that funnel. Most songbirds migrate at night and navigate partly by the stars, and artificial light draws them in and disorients them. Studies of light pollution during migration have ranked Chicago as the most dangerous US city for migrating birds. That October night, bad weather pushed birds toward the lake and forced them to fly low, which made collisions with McCormick Place almost unavoidable.

Glass is the second half of the problem. Birds do not understand glass. They see either the sky reflected in it or the plants visible through it, and they fly straight in at full speed. Estimates put annual building-collision deaths in the United States at roughly 388 to 965 million birds, about 30 deaths every second. These deaths fall on bird populations that are already in steep decline: North America has lost nearly three billion birds since 1970. Collisions are not the only cause, but they are among the most preventable.

McCormick Place had been warned for decades. Advocates said its management had been told for 30 years that it was a major bird killer and that turning off lights would save lives, yet it often chose not to comply. The Chicago Bird Alliance coordinates a voluntary Lights Out program that most downtown buildings follow, and on the night of the disaster, volunteers collected about as many dead birds across all the Loop buildings they monitor as were found at McCormick Place alone.

What happened next shows how solvable this problem is. In 2024, McCormick Place's management spent $1.2 million retrofitting its glass, with the work done between June and September. The building used Feather Friendly window film, a grid of small dots that birds read as a solid surface. By early 2025, bird deaths at the Lakeside Center were down more than 95 percent. One building, one season of work, and the deadliest glass wall in the country became one of the safest.

Turning that one success into citywide policy has been slower. In 2020, the City Council directed the Department of Planning and Development to strengthen bird-friendly design in its Sustainable Development Policy, but that policy applies only to new developments that need the department's approval. Advocates have criticized the city for not making bird-friendly design mandatory, even as the suburbs of Evanston and Skokie have passed ordinances requiring bird-safe glass in new buildings. The tools exist: dimmer lights during migration, patterned glass, and design standards for new construction. Chicago built its reputation on architecture. Every spring and fall, its skyline is tested by birds that have no way to see it, and the city's challenge is to design buildings that pass that test.