Workers at the Chicago Field Museum inspect the bodies of migrating birds that were killed when they flew into the windows of the McCormick Place Lakeside Center.
CNN  — 

In just one night, more than a thousand migrating birds died after crashing into a single building in Chicago, due to what experts say was a deadly combination of migration season, difficult weather, and a lack of “bird-friendly” building measures.

The Chicago Field Museum collected more than a thousand dead birds that had collided with the McCormick Place Lakeside Center, a convention center 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, said Prince. And there were likely more birds that flew away after colliding into a building but later died of their injuries, she said.

“It was overwhelming and tragic to see this many birds,” Prince said. “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 said.

There was a particularly high volume of birds set to migrate south for the winter that night. The birds had been waiting for winds from the north or west to ease their journey. “Those birds essentially piled up,” Prince said. When the right winds arrived on Wednesday, a large number of birds set off for their migration at once. Additionally, “there were foggy and low cloud conditions, which can bring them into confusion with lights and buildings,” Prince said. The clouds likely caused the birds to fly at a lower altitude, bringing them closer into contact with buildings. McCormick Place in particular “is one of the first buildings birds encounter as they move along Lake Michigan,” she said.

McCormick Place in Chicago is seen at evening time on July 13, 2008.

Buildings that leave their lights on overnight, when most birds migrate, are also more likely to attract collisions, she added. “Those lights are a very prominent attraction for birds – almost like a lighthouse.”

But birds continued to fatally crash into the McCormick Place Lakeside Center even 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.

And many of the birds that crashed were likely young and embarking on their first migration, according to Prince. “For some of them, this is the first time they encountered a city or an urban area,” she said.

Andrew Farnsworth, an ornithologist at Cornell University who studies bird migration, told CNN in an interview that the collision event was “of enormous magnitude.”

The number of birds killed at McCormick Place during that one night is around the same as would typically die from collisions at the building in a year, according to Farnsworth.

It’s “a very rare thing and a pretty unfortunate thing,” he said.

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 acknowledging the deaths of “an extremely large number of migratory birds.”

“The well-being of migratory birds is of high importance to us, and we are truly saddened by this incident,” the statement reads. Lights were on at the facility due to an event and were turned off as soon as the building was unoccupied, the statement added.

Both Farnsworth and Prince said the McCormick Place incident was a particularly grievous example of a mundane occurrence: Birds crashing into buildings, particularly those with large glass panels that keep their lights on overnight, during the height of the migration season.

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

A 2019 report from researchers at Cornell’s Lab of Ornithology estimated that around 600 million birds die every year in the US after colliding with buildings. Chicago was ranked as the most dangerous city for birds during both the fall and spring migration seasons, followed by Houston and Dallas.

“It’s a serious problem,” Farnsworth said, which has contributed to the decline of several species over the past decades.

It’s a challenge “that is particularly troubling because it’s controllable, it’s something we can solve,” he said.

Farnsworth and Prince both highlighted two major interventions that can help reduce the number of avian collision deaths: implementing “bird-friendly” glass and reducing light pollution.

“Bird-friendly” glass has “some kind of fritting or pattern in it, which lowers the reflectivity, makes it visible to birds,” said Farnsworth. New buildings can incorporate bird-friendly glass into their designs, while existing buildings can retrofit their windows with decals that make the glass more visible to birds.

Convincing people to switch over to bird-friendly glass presents its own challenges, according to Prince.

“People are very much in love with the aesthetic of clear or reflective glass,” she said. “I think there has to be a mindset change – that none of those aesthetics are worth hundreds of thousands of things dying because of it.”

Additionally, turning off lights, especially overnight, can be critical in saving the animals’ lives.

“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.”

The impact of collision deaths will only increase as birds and other animals face threats from climate change and habitat destruction as well, according to Prince.

“These birds are not replaceable,” said Prince. “They’re valuable too, because we enjoy them, and they’re valuable because they’re critical to our environment.”