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.