Why did 1,000 birds die in one night last week by smashing into this Chicago building?

One night last week, around 1,000 birds died smashing into the windows of Chicago's McCormick Place Lakeside Center.

"It was just like a carpet of dead birds at the windows there," says David Willard, a former manager of the Chicago Field Museum's bird division collections. "A normal night would be zero to 15 [dead] birds. It was just kind of a shocking outlier to what we've experienced."

What happened? From Smithsonian:

At 3:40 a.m. on the morning of October 5, an unusually high number of birds—about 1.49 million—were in flight above Cook County, Illinois, where Chicago is located, according to BirdCast, an online tool that tracks bird migration. Poor weather conditions in the previous days—including heat and headwinds—had largely halted migration leading up to Wednesday. 

"Birds like to fly in the fall when there is a north or a west wind, because they're coming from areas north of us, and that gives them a literal and figurative tailwind to travel with," Annette Prince, the director of Chicago Bird Collision Monitors, tells NPR.  

On Wednesday night, however, temperatures dropped, and the wind shifted, leading a high number of birds to take flight on their journeys south. But early on Thursday, a storm system moved through the city and forced the birds to fly closer to the ground to avoid it, per the New York Times. By morning, carcasses ranging from Tennessee warblers to hermit thrushes to American woodcocks littered the ground outside the convention center. 

McCormick Place posted a statement about how they are "saddened by this incident" and "are in discussion with industry experts to look for better solutions to protecting our avian neighbors."