What killed these thousands of migratory birds in Chicago?

Experts in Chicago said that thousands of migration birds were killed after colliding with McCormick Place Lakeside Center, a convention centre, even in daylight due to bad weather and a large number of simultaneous movements.

"The Chicago Field Museum collected more than a thousand dead birds that collided with the building located on the shore of Lake Michigan, Wednesday night into Thursday morning," Annette Prince, Director of Chicago Bird Collision Monitors, told CNN.

Volunteers working with Chicago Bird Collision Monitors collected an additional thousand dead birds from the city’s downtown area, added Prince.

The expert noted that there were likely other birds that collided and were injured but later succumbed to the injuries.

Prince said: "It was overwhelming and tragic to see this many birds. I went to a building where, when I walked up to the building, it was like there was just a carpet of dead and dying and injured birds."

A combination of factors likely contributed to the extraordinary number of deadly collisions, Prince stated.

"Those birds essentially piled up and migrated at once,” Prince said adding that "there were foggy and low cloud conditions, which can bring them into confusion with lights and buildings, causing them to fly lower altitude."

The birds continued to crash during the daytime, she said, which highlights the extent to which large panels of clear glass can confuse the animals.

"If you use a large expanse of glass that looks like an open space, birds will try to fly into it, not seeing that barrier between them," she said.

Andrew Farnsworth, an ornithologist at Cornell University who studies bird migration, told CNN: "It’s a very rare thing and a pretty unfortunate thing."

McCormick Place cited 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 buildings," in a statement.

"The well-being of migratory birds is of high importance to us, and we are truly saddened by this incident," the statement read.

Lights were turned off as soon as the building was unoccupied, the statement added.

“The collision problem happens every night of migration in spring and fall,” Farnsworth said.

According to a 2019 report, around 600 million birds die every year in the US after collisions with buildings and Chicago was ranked the most dangerous city.

Farnsworth and Prince suggested implementing “bird-friendly” glass and reducing light pollution.

“Turning off nonessential lights is like a no-brainer,” Farnsworth said. “It saves energy, it’s good for human health, and it stops birds from being attracted and disoriented.”