Nearly 1,000 birds died after flying into a Chicago building on a single day last week, a grisly death toll far surpassing past migration seasons.
Experts believe that an unusually large migration, bad weather and a lack of "bird friendly" features on buildings are to blame for the deaths.
About 960 birds were recovered from the McCormick Place Lakeside Center.
Activists have been calling on buildings to turn off bright lights, which can disorient birds.
The birds were collected by scientists and volunteers at the nearby Field Museum, which monitors the McCormick Place, the largest convention centre in North America, for dead or injured birds.
One of the museum's conservation ecologists, Douglas Stotz, told National Public Radio that "in one night we had a year's worth of death".
Mr Stotz added that between 1,000 and 2,000 birds die after striking McCormick Place each year.
Annette Prince, director of Chicago Bird Collision Monitors. told local radio station WGN that the group found 700 to 800 birds in a single square mile it monitors.
Ms Prince described the number of dead birds as a "very unusual and tragic occurrence".
In a statement posted to Instagram, McCormick Place acknowledged that an "extremely large" number of migratory birds had died "due to unusual weather conditions" and "avian confusion" caused by lights.
Lighting at McCormick Place is normally turned off at night, but had been kept on for an event at the property.
"The well-being of migratory birds is of high importance to us, and we are truly saddened by the incident," the statement added.
Previously, the highest daily total of dead birds recovered at the centre was between 200 and 300 birds, he added.
Experts believe that an abnormally large number of birds were flying in and around Chicago and the other parts of Cook County last Wednesday, taking advantage of low temperatures and favourable winds.
Birdcast, a tracking project by three US universities, estimated that nearly 1.5 million birds were in flight above Cook County on the night of 5 October, when the deaths at McCormick Place took place.
Around the same time, a storm passed over the city, forcing many of the birds to come down to the ground, where they face increased danger from lights and windows.
A 2019 study from Cornell University's Lab of Ornithology estimated that approximately 600 million birds die in building collisions in the US each year, with Chicago, Houston and Dallas the most dangerous cities.
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