Bird Deaths From Window Strikes Are Way Higher Than We Ever Imagined

The death toll for birds that crash into buildings may be underestimated by hundreds of millions per year, according to a new study. Previous research only accounted for birds that died at the scene of a collision, but the true number could exceed a billion deaths per year, the researchers reported.

The reason for the discrepancy is due to flawed methodology in previous studies, which relied on counting the number of carcasses collected from crash sites, the researchers wrote in the study, published in the journal PLOS One. That means only birds who died on impact, or who suffered mortal injuries that left them unable to leave the area before expiring, were counted. While aviary scientists previously believed that around 80% of birds that collide with a building die on impact, some recent research indicates this may not be the case. A 2022 study found that under 10% of birds who struck a building were found dead, with the rest being dazed, or suffering injuries that weren’t immediately fatal.

The researchers behind the new study acknowledged that it’s impossible to know exactly how many of the surviving birds flew off, only to die shortly after. They attempted to close that knowledge gap by looking at the mortality rate of wounded birds that were taken to rehabilitation centers.

“There’s a huge blind spot in those birds that hit buildings and survive, at least for a little while, and looking at rehabilitation data can help remove that blind spot and help us make more informed decisions about conservation and preventing window collisions for bird populations,” said Ar Kornreich, a PhD student at Fordham University who led the research, in a statement.

After examining over 3,150 cases involving birds from 152 different species, the researchers found that only 40% were released back into the wild, with most of the remaining birds either dying during treatment or being euthanized. When they applied that ratio to the number of collisions each year, they calculated the number of deaths per year at over 1.09 billion.

The new death count is almost double some previous estimates. A 2015 study found that collisions with buildings accounted for 599 million bird deaths per year in the United States. According to that same study, cats are by far the leading anthropogenic cause of death for birds in the U.S., accounting for over 2.4 billion deaths annually. Wind turbines, in comparison, account for 570,000 bird deaths per year.

Related article: New NYC Law Would Require New Buildings to Have Bird-Friendly Windows

A number of states and municipalities have enacted laws requiring buildings to be designed to be more bird-friendly. For example, there is bird-safe glass that reduces reflections, and architectural features that minimize large expanses of clear glass to prevent bird collisions. Despite those steps, Dustin Partridge, director of conservation and science at the NYC Bird Alliance, who worked on the study, said if more action isn’t taken, there could be dire consequences for birds, humans, and other animals alike.

“Bird populations across the globe are in steep decline, in large part due to collisions with buildings,” he said in a statement. “Unfortunately, our work shows population-level declines are not something we can fix with rehabilitation. If we don’t address artificial light and glass, we’ll continue to lose over a billion birds in the US annually—a toll our ecosystems can’t sustain.”