Chicago, Illinois - Almost 1,000 birds died last week after flying into a Chicago convention center as the animals were migrating south for the winter.

Late last week, 964 birds flew into McCormick Place Lakeside Center, which is a mostly glass building south of downtown Chicago.

Douglas Stotz, a conservation ecologist with the Field Museum in Chicago, told NPR that between 1,000 and 2,000 birds die each year from flying into the convention center.

"In one night we had a year's worth of death," Stotz said.

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"We check McCormick Place daily during migration to pick up dead birds and to release ones that are still alive," he continued.

"When I went into the museum, it was way beyond what I could have imagined."

Millions of birds fly over Chicago during fall and spring migrations, a few specific conditions converged to make last Wednesday one of the busiest migration nights in the last century.

Bad weather in the previous days prevented birds from taking their migratory flights through the Windy City. This meant more birds were flying Wednesday evening, and a storm to the south of the city forced the animals to fly closer to the ground. All this proved deadly.

Before last week, Stotz said the previous number of birds killed after striking the McCormick Place Lakeside Center was around 200 or 300.

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Conservationists say that light reduction is key to reducing the number of birds that die when flying through urban areas. On Twitter the Field Museum noted, "Illuminated windows can disorient birds, resulting in more collisions."

McCormick Place said in a statement posted to Instagram that they participate in Chicago's voluntary Lights Out program, which asks that tall buildings turn their lights out after 11 PM during spring and fall migration dates.

"Lighting at McCormick Place is turned off unless needed for our employees, clients, or visitors. It is important to understand that there is an event going on at Lakeside Center this week, and thus the lights have been on when the space is occupied," they said.

"It would have made a huge difference to have the lights off," Stotz said.