Nearly 1,000 songbirds fatally crashed into the windows of an exhibition hall Wednesday night in northeastern Illinois, according to reports.

The birds crashed into the Lakeside Center and other campus buildings on the grounds of McCormick Place, a famous convention facility in Chicago, the facility’s management said in a statement Friday on X (formerly Twitter). “Unusual weather conditions during the peak of the Fall 2023 migration season in the city coupled with avian confusion that comes from light emanating from the buildings” reportedly caused the deaths.

“It was just like a carpet of dead birds at the windows there,” David Willard, a retired bird division collections manager at the Chicago Field Museum, told the Associated Press (AP).

33 species —  most of them warblers — were identified across the 964 dead birds, the AP reported. “In 40 years of keeping track of what’s happening at McCormick, we’ve never seen anything remotely on that scale,” Willard said.

The birds were flying south along the Lake Michigan shoreline, buoyed by a tailwind after a waiting period forced by unusually warm September southern winds, the AP reported. They lowered their altitude due to rain in the small hours, which made them face the low-slung, illuminated structures at McCormick Place. (RELATED: Huge Flock Of Blackbirds Suddenly Appear To Drop Dead Mid-Flight, Theories Spread)

Reflections of trees and open skies from glass buildings often trick migratory birds, luring between one hundred million and one billion of them to their deaths every year, per the National Audubon Society. Bird-safe glass, fitted with ceramic frits, serigraphs, or ultraviolet coating, can render glass visible to birds and prevent fatal collisions but is reportedly more expensive than standard glass.

“[W]e are truly saddened by this incident,” the McCormick Place management added. The campus’s six-acre Bird Sanctuary and participation in a Chicago-wide Lights Out Program “has helped reduce the number of bird collisions on campus by 80%.” The campus’s Lakeside Center hosted an event during the week and lights were turned off when a space was unoccupied, the statement noted.

The facility’s management faced backlash on X, with some responders highlighting the need for bird-safe windows.