A thump on the window, if you’re around to hear it. A dead songbird below. Many people seek to prevent this sorrowful scenario by warning birds away with decals or film applied to windows of homes and office buildings. But there’s a big catch, a new study suggests: These products only work if affixed to the outside of the glass.

“People who are buying decals and putting them on the windows, they want to do good, they want to do right by the birds,” said John Swaddle, a professor of biology at the College of William & Mary and an author of the study, published Thursday in the research journal PeerJ. “You do have to take the extra step of putting it on the outside of the window.”

Every year, hundreds of millions of birds die in the United States from flying into glass. Combined with other pressures such as habitat lost to development, climate change and hunting by cats, birds have suffered staggering losses in net population. Since 1970, nearly 3 billion birds have disappeared from the United States and Canada, scientists have found.

Treating windows to make them more visible to birds is one way to help. Researchers and manufacturers have long assumed that these products were more effective on the outside. But placing them there often requires ladders or even scaffolding, so sometimes people just put them on the inside and hope for the best.

Going into the study, Swaddle expected to find at least some benefit in those kinds of cases. His team found none at all. “Really all you’re doing is some interior decorating,” he said.

Well-intentioned consumers can fall into other ineffective approaches, for example sticking one or two bird-of-prey silhouettes to a window, even on the outside. Birds don’t recognize the shapes as predators, experts say, and might simply try to fly around them, hitting another part of the glass. The American Bird Conservancy, a nonprofit group, has reviewed about 200 window materials and treatments for bird safety, mostly types of glass for new construction but also films, decals and others.


“There has to be some kind of pattern,” said Christine Sheppard, director of the glass collisions program at the conservancy. “The spacing of the lines, the width of the lines or dots or whatever you’re using is really critical.” People who live on higher floors with windows that don’t open can sometimes enlist window washers to apply the treatment, she noted.

Turning off lights is another important step people can take to help birds, and not just in windows. Lights draw in and disorient birds. Experts recommend using a motion sensor for outdoor lighting, especially floodlights.

Still, most strikes occur during the day, according to the American Bird Conservancy, and while people often assume that high-rises are largely to blame, homes and low-rise buildings account for the vast majority of collisions.

Many residents don’t realize that birds are crashing into their windows. The animals may fly away before succumbing to their injuries, or a cat might grab the body before a person discovers it. During the pandemic, reports of bird strikes increased sharply because people were suddenly at home to see or hear more of them.

To study the effectiveness of placing material on the inside versus outside of windows, Swaddle and his team used zebra finches, protecting them from impact with a fine mesh in front of the glass. The domesticated songbirds see and fly like wild ones, he said, but provide more realistic results because they aren’t panicking when released. When used on the outside of glass, the two products they tested increased window avoidance by as much as 47%. Inside, they were ineffective. One was invisible to humans and the other used a pattern of tiny orange and black diamonds.

The project received some funding from a company that makes one of the window films that was tested. It had “no role in study design, data collection and analysis, decision to publish or preparation of the manuscript,” the authors wrote.

“The study is solid,” said Scott R. Loss, a professor of ecology at Oklahoma State University who has studied birds and window collisions extensively and who was not involved in the research. In the past, when homeowners and property managers have asked if they can apply treatments to the inside, he has urged against it but has never had a peer-reviewed study to support his position.

“This is a really good contribution that we’ll be able to point to now,” he said.