Thud.

It's a stomach-turning sound to anyone who has seen or heard a bird fly into a window at full tilt. Birds can't distinguish glass from open air and that lack of judgment can injure or kill them.

No one knows how many birds whack into windows each year, but researcher Daniel Klem Jr. put the U.S. death toll at 100 million to 1 billion. That's as much as 5 percent of the bird population after the breeding season, and he suspects the actual number may be even higher.

"Almost every structure on this planet has a piece of glass in it," he said, and each of those windows is a potential hazard to birds.

Klem, an ornithologist at Muhlenberg College in Allentown, Pa., based his estimate on research indicating one to 10 birds are killed each year for each building in this country, on average. And that doesn't count all the birds that end up with sore heads or battered beaks from their nonfatal run-ins.

The problem, Klem said, is that birds don't perceive glass as an object. They see the sky reflected in a window, or they spot habitat through another window beyond, and into the glass they go.

Bird watcher Bill Thompson III has seen as many as five birds a day killed against the windows of his house, which sits on a ridge in a rural area in southeastern Ohio. His wife has a windowed studio that overlooks a brushy area, and especially during spring when the young birds are starting to fly, "we get thunk, thunk, thunk, thunk," said Thompson, editor of the magazine Bird Watchers Digest and a director of the Ohio Ornithological Society.

About a quarter of North American bird species have been known to fly into windows under a variety of circumstances, Klem has written. Some are trying to outrun a predator. Some are migrating. Others are looking for food, water or shelter.

Most strikes happen in winter, he said, when birds visit feeders in large numbers and often run into windows on their way to or from feeding stations. The problem gets the most attention during spring and fall migrations, however, because window accidents and their victims are most noticeable then.

Mating season is a time for a less dangerous kind of bird strike, when a male bird, often a cardinal or robin, sees a reflection of itself and think it's competition, said Damon Greer, a wildlife biologist with the Ohio Department of Natural Resources' Division of Wildlife. The bird will peck at the glass persistently to try to get at the intruder, sometimes for weeks on end. The bird rarely suffers anything worse than the occasional bloodied bill, Greer said, but the humans on the other side of the window can be driven to distraction.

Klem has argued for changes in building design to reduce bird strikes, including angling first-floor windows downward so they reflect the ground instead of the sky. So far, he said, he's gotten little heed from the building industry.

That could be changing, though. Audubon magazine wrote in its March issue that Klem's work has influenced construction projects including a science building at Swarthmore College, which will have windows with dots of opaque glass to discourage bird strikes. And the New York City Audubon Society is campaigning for bird-friendly design in any buildings that rise on the site of the former World Trade Center.

In the short term, however, challenges remain. Thompson, for example, tried to have windows installed at a downward angle on a construction project at his farm and discovered that doing so would void the manufacturer's warranty.

Bird experts offer other suggestions for preventing bird strikes, such as stretching netting over windows to cushion the blow or hanging objects just outside to break up the birds' view. None is foolproof, however, and homeowners often object if they obscure the view, block the sunlight or look unattractive.

Still, Thompson believes they're useful at least as short-term measures for times when bird strikes are prevalent, such as migration.

"There are some drawbacks to every single one of them," he said. "You have to balance utility with making them [windows] as safe as possible for birds."

Here are some methods authorities recommend to discourage birds from hitting windows:

* Move your feeders. If fewer birds are encouraged to congregate near your windows, the odds of one running into the glass will drop. Klem recommended locating feeders, birdbaths and other attractants far away from windows.

If that's unacceptable, place them no more than three feet from the window, Klem said. Birds slow as they approach feeders, he explained, so if a feeder is close to a window, a bird is less likely to be hurt badly if it hits during approach. And if a bird is startled away from the feeder and immediately runs into a window, it won't have built up enough speed for serious injury.

* Break the impact. Insect screening or fine netting installed over the exterior of a window intercepts the bird's flight into a window, Klem said. It can either prevent the bird from hitting the window altogether or slow the bird down and lessen the blow.

* Break up the view. Stickers affixed to the outside of a window interrupt a bird's field of vision, Klem said, so the bird no longer perceives the window as a stretch of open sky. But forget the idea of putting up one or two stickers of hawks, owls or other predators, he said. For one thing, birds don't perceive the stickers as animals. For another, the stickers don't cover enough of the window to be effective.

Instead, Klem said stickers need to cover the whole window uniformly and be positioned close together, ideally, two to four inches apart. The shape of the stickers isn't important, he said; the amount of window covered is. You can use fewer stickers and space them farther apart, he said, but they won't work as well.

Affixing strips of tape to the window in a grid pattern has the same effect, wildlife biologist Damon Greer said. Installing imitation window mullions can help somewhat, too, he said.

Greer and Bill Thompson III, editor of Bird Watchers Digest, said hanging a plant directly outside a window or suspending something like tree branches over the window's exterior works in a similar way to break up the view. Klem, however, said the real benefit is that a plant or branch invites a bird to land there, so the bird slows its flight.

Thompson has also had some luck with bird feathers strung about 8 inches apart on fishing line, an idea that came from a reader of his magazine. Thompson isn't sure whether the feathers' movement deters birds or whether the mass of feathers signals trouble and keeps them away. All he knows is that the device has worked at his house.

* Reduce reflection. Screening and netting have the added benefit of reducing a window's reflectivity, Thompson said. A light coating of vegetable-oil spray or even spray-on fake snow will do the same thing, he said, as will plastic food wrap stuck to the window -- if necessary, on top of a light spray of vegetable oil or water.

Paul Wright, owner of the Wild Bird Center in Fairlawn, suggested window tinting designed to reduce ultraviolet rays can cut reflectivity as well, and Klem said the idea has promise. For it to be truly effective, however, Klem said the tinting material would need to be manufactured with some kind of pattern visible from the outside that birds would recognize -- like the one-way film used for advertisements on bus windows.

Whatever covering you use to reduce reflectivity, it needs to be applied to the outside of the window, Wright noted.

* Dazzling alternatives. Birds dislike shiny things, Wright said. An economical approach is to attach a Mylar balloon to the window sill, or suspend shiny objects such as foil strips, aluminum pie pans, old compact discs or Christmas ornaments outside the window.

Klem, however, discounts those sorts of methods. If they have an effect, he said, it's because they break up the bird's field of vision.

They're not the most attractive solutions, either, and Thompson said the clatter of CDs hitting your glass might make you rethink the idea.

* Install translucent glass. In a problem situation, you might consider replacing transparent glass with frosted or fritted glass, Klem said. Or try this tip he picked up at Chicago's Lincoln Park Zoo: Soap the outside of a problem window during the time of year when bird strikes are high. Soap film doesn't reflect the way glass does.