At least 960 migrating birds, the highest number on record, died Thursday in “massive carnage” at McCormick Place Lakeside Center, according to David Willard, a retired bird division collections manager at the Field Museum.

Birds were crashing into windows even as monitors collected the casualties, Willard said.

“It was just discouraging as can be,” said Willard. “You’re looking at a rose-breasted grosbeak that, if it hadn’t hit a Chicago window would have made it to the Andes of Peru.”

Willard blamed the worst day in 40 years of monitoring on an array of factors, including weather patterns, badly timed rain and lit windows at Lakeside Center.

[ Turning off just half the lights at McCormick Place could reduce the mortality rate of migratory birds by 60%, study shows ]

First, there was a stretch of time with few winds out of the north, which left a lot of birds backed up and ready to migrate: “Sometimes, it’s like ‘now or never,’ and they go,” said Willard.

Then, when the flight began, it was huge. One local birder who was out Thursday morning told Willard that he hadn’t seen anything equal to it in 40 years. Another birder said he saw 100,000 birds move past Promontory Point in Hyde Park. Wrigleyville resident Nathan Goldberg told the Tribune he saw roughly 172,000 birds overhead in under four hours.

Rains early in the morning may have driven the waves of birds lower to the ground — and closer to danger.

And then there were the windows at Lakeside Center. It’s estimated that a billion birds die crashing into windows in the United States every year, Willard said.

Thursday was also a record day for Chicago Bird Collision Monitors, which found about 450 dead birds and rescued 300 injured birds, according to director Annette Prince.

“We’re talking about irreplaceable birds that are a critical part of a healthy environment,” Prince said. “They’re already declining significantly and for them to die needlessly at the feet of all these buildings as they did today — it’s a tragedy.”

[ As many as a billion birds are killed crashing into buildings each year — and Chicago’s skyline is the most dangerous area in the country ]

There are fixes for bird-building collisions, Willard said, including bird-friendly windows, but the issue tends to attract more attention than action. In Chicago, a City Council ordinance requiring bird safety measures in many new buildings passed in 2020, according to Prince, but has not yet been implemented.

The dead birds at Lakeside Center were photographed and taken to the Field Museum, where they will be prepared as scientific specimens and used in studies.

Many were palm warblers and yellow-rumped warblers, colorful little songbirds that are common in Chicagoland in spring and fall.

Willard said that monitors at Lakeside Center generally find zero to 15 dead birds on an average day.

In the worst days of the 40-year monitoring project, 100 to 200 birds were found dead.

nschoenberg@chicagotribune.com