A groundbreaking study published in PLOS One on August 7, 2024, reveals that over one billion birds may die annually in the United States due to building collisions—a number potentially underestimated by hundreds of millions.
The study underscores the significant impact of these collisions with glass windows, which often result in delayed fatalities not accounted for in traditional estimates.
Led by Ar Kornreich from Fordham University and co-authored by researchers from the NYC Bird Alliance and the Max Planck Institute, the findings highlight that nearly 60% of birds brought to rehabilitation centers after collisions ultimately die, a stark contrast to previous assumptions that these birds often recover and survive long-term.
This study emerges against a backdrop of alarming declines in bird populations across North America. A 2019 study published in Science revealed that nearly 3 billion birds, or 29% of the bird population, have been lost since 1970. This decline spans most biomes and includes many once-common species, reflecting the broader biodiversity crisis exacerbated by habitat loss, climate change, and other human-induced factors.
This new understanding challenges existing estimates of bird deaths due to collisions and calls for more comprehensive conservation strategies to protect vulnerable bird populations.
The researchers analyzed data from over 3,100 bird-building collision cases in the Northeastern United States, spanning multiple seasons and involving 152 avian species. Their findings reveal that while fall migration sees the highest number of collisions, summer collisions result in the lowest survival rates. This seasonal variation, coupled with the type and severity of injuries sustained—most commonly head trauma and concussion—contributes to the high mortality rate among affected birds.
The danger posed by glass to birds lies in its deceptive appearance. Unlike humans, who learn to recognize glass as a barrier, birds perceive glass as a continuation of their natural environment. Reflections on glass surfaces, such as open sky, vegetation, or other natural elements, can create the illusion of open space, leading birds to fly directly into it at high speeds.
Additionally, glass can appear transparent, causing birds to mistakenly believe they can fly through what they perceive as an open passage. This misunderstanding often results in severe injuries, such as head trauma or concussions, which significantly reduce the likelihood of survival.
“Building collisions are one of the leading anthropogenic causes of bird mortality in the United States,” the study’s authors wrote, emphasizing the severity of the threat. “Communication between the fields of scientific research and wildlife rehabilitation is critical for increased understanding of the building collision crisis, and subsequently how to mitigate its impact on bird populations.”
Despite efforts in some cities to implement bird-friendly building designs, the U.S. Fish and Wildlife Service has yet to take decisive action on a national scale.
The authors found that larger birds have a better chance of survival post-collision, with mass positively affecting their likelihood of release from rehabilitation centers. In contrast, birds injured during the summer months were less likely to survive, possibly due to the increased vulnerability of juvenile birds during this season.
The study’s findings are consistent with an earlier estimate that nearly one billion birds collide with glass annually. The U.S. Fish and Wildlife Service has promoted various measures to reduce these collisions, including the use of bird-friendly glass and reduced nighttime lighting. However, the PLOS One study suggests that current efforts may be insufficient to fully address the scale of the problem.
“This heartbreaking study drives home how much more must be done to truly protect vulnerable birds from window collisions,” said Tara Zuardo, a senior campaigner at the Center for Biological Diversity who was not involved in the study. “It’s time for federal officials to step up to protect our birds.”
The study adds urgency to ongoing conservation efforts, as its authors call for increased collaboration between wildlife protection agencies and organizations and conservation researchers to better understand and mitigate the long-term effects of building collisions on bird populations.
The study’s authors also advocate for a reevaluation of current mortality estimates and call for stronger conservation policies to mitigate the impact of building collisions on birds, including by making glass visible to birds and reducing artificial light at night should remain top priorities.
“Prevention of building collisions by making glass visible to birds and reducing artificial light at night should remain the top priority reducing the impact of collisions, and wildlife rehabilitators should continue to educate the public and advocate for implementation of safety measures,” they said.
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