THUD.
When a bird smacks into a glass window, the force is nothing short of shocking. Panes rattle. Bones break. Brains bleed.
However, only a small percentage of birds die upon impact, with many regaining consciousness and flying off wounded, while an unknown number of those are carried off by predators. This makes the phenomenon hard to study.
Now, new, more in-depth research shows we may be severely underestimating the annual death toll of U.S. birds crashing into glass.
Previous research had relied on counting birds found dead next to windows and other glass structures. But the researchers took that a giant step further by observing birds in real-time for five years, in addition to studying bird-rehabilitation data.
The results, published recently in The Wilson Journal of Ornithology, suggest between 1.28 billion and 3.46 billion birds die in glass collisions each year. That number, which represents a 350 percent increase over the previous estimate in 2014, is just for the United States—meaning the global impact is likely many, many times higher.
“The whole bloody world is filled with glass right now,” says study leader Daniel Klem, an ornithologist at Muhlenberg College in Pennsylvania.
Dead male and female scarlet tanagers lie in the grass after striking a glass window.
Photograph By Melissa Groo, National Geographic Image Collection
The global glass construction market reached $110 billion in 2023, and is projected to grow to $177 billion by 2032, according to industry estimates. (Read how better glass can save hundreds of millions of birds a year.)
“Birds around the world really are taking a big hit, literally, and the consequence is that we’re losing a tremendous number of the population,” he says, pointing to a recent study that estimates three billion birds have been lost in North American since 1970, due to factors such as pesticides, habitat loss, and outdoor cats.
Through the looking glass
For the research, Klem and colleagues logged more than 1,200 hours of observation over five years. Using birdseed, they attracted birds to a forest edge in Henningsville, Pennsylvania, then watched how the animals behaved near a row of experimental framed window units placed about 30 feet away from the bird feeders.
Curiously, of the more than 1,300 birds that struck windowpanes, 50 percent of them left no mark, such as feathers, dust smudges, or blood. This alone suggests that many of the costs to birds from window strikes are going unnoticed, the authors say. (See six ways you can help birds in your own home.)
Overall, the researchers found that only 14 percent of collisions resulted in an immediate fatality. However, a subset of experiments also showed that another 14 percent of birds were left either unconscious or stunned for five minutes or longer after striking the panels, before flying off and possibly dying later.
“We don’t like that. Nobody likes that,” Klem says of the birds who died during the study.
However, he stressed that such studies are necessary to identify the risks birds face in the real world and develop products to minimize them. The Muhlenberg College Institutional Animal Care and Use Committee approved the experimental protocol, and the U.S. Fish and Wildlife Service and Pennsylvania Game Commission issued permits.
Separately, the scientists gathered data from 10 animal-rehabilitation facilities across the U.S. Northeast and Great Lakes regions. Of the nearly 9,000 birds brought to a facility because of a collision with a window, 70 percent ultimately perished. (Learn how record numbers of birds are being rescued in NYC glass collisions.)
An invisible killer
For the last half century, Klem has been one of the scientists leading the charge in the field of bird-on-glass collisions.
“Ninety-nine percent of all the windows in the world are reflective,” says Klem, who also wrote Solid Air: Invisible Killer—Saving Billions of Birds from Windows. “Even a perfectly clear pane covering a darkened interior space acts like a mirror on the outside.”
This leads to collisions when that surface reflects trees or the sky—the birds simply think they’re flying toward more habitat, not a solid material.
The species affected are diverse, as well: In Klem’s Pennsylvania study, more than a dozen species of birds struck the experimental windows, such as mourning doves, downy woodpeckers, red-winged blackbirds, northern cardinals, dark-eyed juncos, and Cooper’s hawks.
“It’s good to highlight how dangerous windows are,” says Luke DeGroote, who was not involved with the research, but reviewed an earlier version of the study. DeGroote also praised the study’s finding that half of collisions fail to leave a mark, something he’s witnessed anecdotally but never seen acknowledged about in the scientific literature.
However, DeGroote, who is the lead avian research coordinator at the Carnegie Museum of Natural History’s Powdermill Nature Reserve in Rector, Pennsylvania, cautions about the reliability of the study’s calculations.
For instance, the statistics from wildlife-rehabilitation facilities have not undergone peer review, he noted. This doesn’t mean the data is incorrect, but DeGroote knows from his own research that rehabilitation success rates can vary widely.
“I know one of my rehabbers has a phenomenal success rate of 90-plus percent,” says DeGroote. “So 70 percent mortality seems high.”
How to protect birds
The good news is that people can easily prevent birds from slamming into windows.
“This is not complex stuff, right? It’s not climate change. It’s not multifaceted unknowns,” says Klem. “We know how to solve this problem.” (Related: “Four ways to help your local wildlife this spring.”)
Both Klem and DeGroote run experiments that test new products—such as films, coatings, patterns, and decals—that can be placed on glass to make it more visible to birds.
And while sports stadiums and skyscrapers may require advanced treatments—and political will—there are plenty of inexpensive products bird-friendly consumers can install in their homes.
For instance, hanging cords over windows, or covering them with adhesive squares or translucent tape, can deter birds from striking windows.
“We could [save the birds] tomorrow, if people were willing,” says Klem.
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