In the United States alone, well over 100 million birds are killed each year, not by hunters, not by wind turbines, not by cats, but by the invisible killer – glass windows.

"This is probably the most ignored environmental problem there is," said Lehigh Gap Nature Center director Dan Kunkle. "It is the biggest human-caused killer of birds in the world, and it is ignored. People don't want to know about it."

"Everybody knows that birds hit windows," he continued. "It's happened to everybody."

With the proliferation of large-paned windows and bird feeders, birds, especially those that are young or migrating, gather around a feeder. Then, especially when the sun plays on the glass, the birds are confused by the window's reflection of the feeder and forest, and fly into it.

As this article was being prepared, there was a thump. On the window were two feathers. A bird feeder was nearby.

"Many birds fly into a window, and people assume they flew away and assume it is O.K.," Kunkle said. "but many times, there's a concussion and the bird is bleeding internally. Even though it was able to fly away from the window and up into a tree, later on, it dies."

If you see a bird fly into a window and it is stunned, Kunkle advises, "Go out and pick up the bird, and put it in the show box. Keep it dark, which keeps it quiet and calm. If it's going to recover, that's the best way to help it recover. Give it a half hour or so, take the shoebox back outside if it was cold and it was taken inside. Open it up. You will either have a dead bird in the box, or a bird that's ready to fly away. If it flies away, then it probably has a good chance of surviving."

The Lehigh Gap Nature Center is nearing the completion of its expansion. "Almost as soon as we built the building, we put those decals on the windows, but we have had birds hit other windows that we didn't have material on."

Kunkle also cited birds striking the windows of school buses outside the building. "They don't recognize glass as a substance. They see through it, or they see a reflection on it and think they are flying into whatever that reflection is."

The Center purchased a material marketed to prevent bird collisions with windows called CollidEscape, a perforated vinyl sheeting that goes on the outside of windows. The side towards the interior is colored black and coated with a peelable adhesive. The side facing the outside is a white printable surface. From the inside, the material looks like a mesh screen. From the outside, the material appears opaque and non-reflective.

At the Center, Kunkle had the material cut into the shape of raptors-American kestrels and merlins. This does not help to keep the birds away from hitting the windows. It is done to attract attention to the material and to illustrate the silhouettes of birds as a teaching tool.

"People think it is the shapes that chase the birds away, that the birds see the raptor shapes and it keeps them away from the window," Kunkle explained. "That's not the case at all. What keeps the birds away is it takes away the reflections from the glass, so they don't see the reflection. It doesn't look like they can fly through it. The shapes were simply for a teaching tool. It makes no difference to the birds."When applying a perforated window covering material, it is best to cover the entire window. In sections as small as a couple of inches between the raptor decals, Kunkle noted, "A chickadee hit that window."

Kunkle suggested that bird feeders be placed either as far away as possible or close to a window. "If the feeder is close enough to your window, the bird doesn't get enough speed to kill itself or hurt itself badly."

Other suggestions to reduce the likelihood of a bird hitting a window are: install screens, close drapes or blinds, draw or etch lines in the glass, hang ornaments on windows with a suction cup, cover windows with netting, paint the outside of windows, install exterior sunshades, outside the window hang ribbons or hawk silhouettes that can blow in the wind.

One additional feature of a material such as CollidEscape is that it reduces the UV light and increases privacy. That is why this material printed with an American flag or an outdoor scene is a popular covering on pickup trucks.

The CollidEscape material is somewhat costly at about $3 a square foot-but Kunkle feels that it is effective. "If we entirely covered every window at the Center, I believe we would have 100 per cent effectiveness. I don't think a bird would ever hit a window that was covered with this."