A certain type of bird deaths became the subject of study for Daniel Klem Jr. in 1974 when he earned his doctoral degree from Southern Illinois University. He had intended doing his paper on birds of prey and how they capture food.

A professor told him there might be a project if he sat in front of a building known to be a danger. He saw a mourning dove fly into the window and then found the skeletons of many more birds.

"I was hooked," he said.

His work began by writing to museums that collected the dead birds.

Klem is the Sarkis Acopian Professor of Ornithology & Conservation Biology at Muhlenberg College, Allentown.

Bird deaths from wind turbines and communication towers are estimated at 400,000 a year, 60 million from vehicular collisions, and 120 million from hunting. Cats also account for hundreds of millions of bird casualties.

Destruction of habitat is the chief problem faced by birds as well as other types of wildlife. But second place on the list is something that is seldom discussed – the millions of birds, possibly billions, that are killed by flying into clear or reflective glass.

They die from head trauma when they hit the glass.

The most conservative estimate of 100 million would be the equivalent of 333 Exxon Valdes oil spills, and those birds received a lot of publicity. Perhaps a more exact count is the 34,000,000 killed in only the urban areas in Canada and the United States.

Birds act like clear glass is invisible. The habitat appears to continue through the glass. Reflective glass, like a mirror, repeats the vegetation the bird has just flown through.

"It's been tough over the years. There is something esthetically powerful in glass and the wonder it brings into our lives," Klem said.

Surprisingly, more birds are killed when they congregate around feeders. The bird feeders are placed close to a house so the people can see and enjoy them. A bird that hits glass from as little as a meter (a yard) distant can possibly be killed.

Although the deaths are found in both the plentiful and scarce species of birds, the worst case is the Swift Parrot, a threatened species. There are only 1,500 that breed in Tasmania and winter in Australia.

"Thirty killed by windows is a terrible attrition," Klem said.

The CNN building is mostly glass.

"No one tells (of the deaths) because it is not complimentary," he said.

Nearer to home, the former Lucent building along Route 22 – "a glass palace" – is a deadly site for birds.

In Toronto, which is considered a progressive city, architects have to take into account the danger to birds. Quantitative analysis provides information for professionals so they can better mitigate or eliminate avian mortality due to glass.

An article Klem co-authored about architectural and landscape risk factors said, "the hazard that glass poses to birds is expected to increase as current urban areas increase, and human structures elsewhere are constructed in avian breeding and non-breeding areas and across migratory routes worldwide."

There is a practical use for the birds that are collected as window casualties. They provide information about migratory routes, the expansion and contraction of breeding and non-breeding ranges and as subjects for specimen-related studies.

Until 2003 most literature was published in peer-review scientific journals which were not seen by the majority of people. The subject is now receiving a little more publicity.

Methods of preventing strikes have been found in studies done in flight cages where various types of glass or sheet plastic were used to obstruct one side of the cage while the second side remained unobstructed.

A bird was released and it was recorded if the unobstructed route was taken. If so, it was aware of the obstructions in that particular test.

Things that can be done at residences include the following:

Ÿ Reducing the amount or height of ground cover adjacent to buildings;

Ÿ Covering windows with netting;

Ÿ Moving bird feeders and watering areas to within less than 1 meter;

Ÿ Placing decals or hang strings of objects in front of windows, covering the surface separated by four inches in vertical columns or 2 inches in horizontal columns;

Ÿ Coating windows with one-way films so birds cannot see in;

Ÿ Reducing the percentage of glass in new construction;

Ÿ Angling windows 20 to 40 degrees off the vertical in new construction;

Ÿ Applying bird-safe glass as suggested by Leadership in Energy and Environmental Design.

The Migratory Bird Treaty Act of 1918 and the Endangered Species Act of 1973 provide tools to protect native birdlife. Klem realizes it is unrealistic to expect law enforcement to bring action against everyone with a house where a few birds die from window collisions.

However, he believes the laws should be enforced in locations where hundreds of birds die in a single day.