Now a report on birds and glass. One ornithologist says at least a billion birds are killed every year by flying into windows and glass doors. Now one college is trying to do something about this problem. NPR's John Nielsen traveled to a wet and windy forest in Pennsylvania to find out more.

JOHN NIELSEN reporting:

Nobody knows what birds see when they look out at the world, but ornithologist Daniel Klem says he's pretty sure they don't see glass. If they did, the sickening thud of birds hitting windows wouldn't be such a familiar sound.

Professor DANIEL KLEM (Muhlenberg College): It's a very common phenomenon. Birds are deceived. They just don't see glass as a barrier, and this is a problem for them.

NIELSEN: Klem, a professor at Muhlenberg College in Allentown, Pennsylvania. Thirty-some years ago, one of his teachers mentioned that little was known about the so-called window hit problem. Klem responded by marching into a nearby forest and hanging a bunch of windows from the branches of trees. Then he sat down and watched what he calls an appalling number of collisions. Some birds lived. Many didn't.

Prof. KLEM: I've learned very early on a really short flight from a perch, less than eight feet, and, you know, just a little over a meter away can result in death.

NIELSEN: Later, Klem watched more than a hundred birds hit the windows of a typical suburban home in the course of a single year. Then he studied shiny glass office towers. They can kill tens of thousands of birds in a year. Add it all together and you get a yearly death toll of at least a billion, he argues. That's roughly 5 percent of all the birds found in the United States each fall. Hardly anybody appreciates the size of this problem, he says.

Prof. KLEM: Nobody intentionally wants to harm these animals, and it's so tragic to see this underappreciated source of mortality being ignored.

NIELSEN: Klem says attempts to find solutions to the window hit problem have been few and relatively modest. One big exception can be found here on the campus of Swarthmore College. It recently agreed to turn a $71 million science center into an experiment by using it to test a new kind of glass designed to ward off oncoming birds.

(Soundbite of door being unlocked)

NIELSEN: Julie Hagelin, an ornithologist at Swarthmore, says this college has known for years that it has a bad bird hit problem. To underline that point, she takes us into her lab. Hagelin is the official keeper of dead birds found by people on campus.

Ms. JULIE HAGELIN (Swarthmore College): Typically, these birds are found during the morning when people are coming to campus. People walk to wherever building they're entering for the morning and they find a dead specimen. They wrap it up in whatever they happen to have, a napkin or a plastic bag, and they bring it to me and say, `Here. We heard that you're supposed to do something with these.'

NIELSEN: While she talks, she opens up a big metal cabinet and pulls out an old wooden tray. It holds a long line of cold dead birds arranged according to size.

Ms. HAGELIN: And you can see that they range from everything from very colorful migratory birds, Northern Parula, to more common residents like woodpeckers.

NIELSEN: Many birds killed by windows get eaten by cats and other scavengers, she says, but even when there isn't any carcass, you can usually tell when there's been a collision. That's because most birds leave a distinctive smudge mark on the window. Think of tiny snow angels.

Ms. HAGELIN: When it impacts, its wings sort of spread out in almost an angellike format and you get this little sort of whitish dusty looking imprint which is the result of oils and dandruff and stuff on the bird's feathers that sort of end up on your window.

NIELSEN: In the mid-1990s when Swarthmore decided to build a big new science center on its campus, it briefly seemed like those window marks were about to get a lot more common. The new center was to feature a three-story meeting room made out of clear glass, an avian slaughterhouse in Daniel Klem's words. Alarmed ornithologists invited Klem to speak to the building committee. Carr Everbach, a member of Swarthmore's engineering faculty, says that speech and an unfortunate encounter with a bird helped change the building plan.

Mr. CARR EVERBACH: We were about to have a meeting to go in to talk about how to stop the problem of birds striking windows, and we heard a loud thump. And we turned around and there was a bird sort of flopping around on the pavement just outside the window.

NIELSEN: Everbach and Klem eventually received an offer from the building committee: Design us some bird-friendly glass and we will pay to have it special ordered. The challenge was to find a kind of glass that the birds could see and the people could see through. Everbach and Klem eventually recommended the use of so-called fritted glass, etched with closely spaced rows of small circles. When it's right in front of you, its glass is hard to see through, but when you take a few steps back, it's easy. Everbach thinks it looks great in Swarthmore's new science center.

Mr. EVERBACH: Most people don't even notice the dots. They look up and they see the sky. I can see a leaf blowing through the dotted panes and it's true that the dots are there but they don't obscure my vision very much.

NIELSEN: To find out whether the fritted glass was really bird friendly, Everbach outfitted some of the windows with ingenious video sensors. He calls them thumpers.

Mr. EVERBACH: So that when there's a thump...

(Soundbite of thump noise)

Mr. EVERBACH: see a little LED lights up. It connects with a wire to a wireless transmitter. The transmitter transmits the information that a thump has occurred to a camera looking at the window so that we can scrutinize the video clip to see if a bird hit and what kind of bird and how fast it was going.

NIELSEN: Everbach says his thumpers have recorded just two bird hits since this center was finished five years ago. That doesn't prove anything conclusively since the sensors aren't attached to every window, but Everbach says it's a pretty good hint that the fritted glass is working.

Mr. EVERBACH: However, I don't want to oversell the results. What I would rather say is that we've begun to learn some things and we'd like help from other people whether it's glass companies, research organizations and the public who, if they're interested enough in this to want to pay a little bit more for a pane of glass that's bird dissuasive, then that's useful information.

NIELSEN: Everbach's dream is that one day soon a glass manufacturer will start selling bird-friendly glass just as the fishing industry now sells cans of so-called dolphin-safe tuna.

Mr. EVERBACH: If it were available for people at a small incremental price at Home Depot to buy something that worked and to keep birds from flying into the glass, I think people would do it.

NIELSEN: Klem says he shares Everbach's dream, but after spending 30 years fighting this problem, he's learned one crucial thing.

Prof. KLEM: If you're going to come up with a solution to this problem, you're going to have to come up with one that doesn't muck up the way people look out their windows. You start playing around with the way they see the world from their homes or their workplaces, they're going to be unsympathetic.

NIELSEN: If nothing changes, Klem says it's certain that the problem of window hits will escalate. He notes that in the next few decades, millions of new homes and offices will be built in the United States alone. John Nielsen, NPR News, Washington.

MONTAGNE: Tips to help birds avoid your windows are at

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