Hundreds of migrating songbirds fatally crashed into New York City’s glass buildings in the World Trade Center. 

At least 291 birds were found dead on the city’s sidewalks after becoming disoriented by the skyscraper’s reflective glasses. Black-and-white warblers, American redstarts and ovenbirds were some of the species found in the flock’s graveyard. 

Kaitlyn Parkins, associate director of conservation and science at NYC Audubon, told AP News that the glass buildings combined with the stormy weather Monday and Tuesday contributed to the deaths.

“We had a big storm and sort of weird weather and lots of birds, and that’s sort of the perfect combination that can lead to bird-window collisions,” Parkins said.

“It seems that the storm might have brought the birds in lower than they would have otherwise have been, or just disoriented them,” she added. “The effects of nocturnal light on birds is also quite strong, especially when it’s a cloudy night.”

Parkins said bird strikes on the city’s skyscrapers are a recurring problem and NYC Audubon has been pressing for owners of the World Trade Center towers to dim building lights at night and treat the glass for the birds’ visibility to prevent deadly incidents such as this week’s.

A study by Portland Audubon found that about one billion birds die each year in the U.S. due to window collisions, of which up to 76% are fatal.  

Jordan Barowitz, a spokesperson for the Durst Organization, co-developer of One World Trade Center, said in an email that the first 200 feet of his building consists of non-reflective glass fins. 

“This design was chosen because it greatly reduces bird strikes which mostly occur below 200 feet and are frequently caused by reflective glass,” Barowitz added. 

There are two main types of window collisions: daytime and nighttime. According to the Cornell Lab of Ornithology, birds crash into windows during daylight because they see reflections of vegetation or they see through the glass. Meanwhile, nocturnal migrants, which includes most songbirds, are diverged from their migration paths by light reflection, especially in low-ceiling or foggy conditions. 

Melissa Breyer, the editorial director of the environmental publication Treehugger, said on Twitter that she spent over an hour before sunrise picking the dead birds and sharing pictures to bring attention to “the tragedy of migratory songbird deaths.”

Of the hundreds of dead birds, about 77 were sent to the Wild Bird Fund’s rehab to be treated for injuries, director Ritamary McMahon told AP News.