Tall buildings, whose tenants often leave signage and office space brightly lit at night, have long been recognized as a danger to migrating birds. Nobody knows how many perish after being drawn to the light and die from the impact of a collision or from exhaustion, but most estimates are in the tens of thousands in the United States each year.

Dr. Daniel Klem, a biology professor at Muhlenberg College in Allentown, Pa., estimates the number of birds killed in the United States each year by colliding with buildings could be as high as one billion.

"I think that's a conservative estimate," he said. "Birds just don't see glass."

Volunteers who patrol city sidewalks in the early morning during migration routinely return with a handful of dead or injured birds, according to experts.

But the observable casualty rate is just a small fraction of the total because the number of volunteers is dwarfed by the volume of birds.

DEATHS REDUCED

Migrating birds can die not only from the impact of flying into plate glass they do not recognize as a hazard but from exhaustion after flying incessantly around a source of light to which they are drawn.

Dazed and disoriented birds not killed on impact may be snapped up later by predators such as crows and sea gulls that have learned to regard such sites as rich hunting grounds.

Bird-watchers report that efforts to reduce the amount of distracting artificial light significantly lowers the number of birds killed.

Chicago leads U.S. efforts to reduce migration casualties. About 30 major city-center buildings turn out their lights.

At McCormick Place, a large lakefront convention center, ornithologists from the city's Field Museum found that bird mortality declined by about 80 percent over a two-year period.

Before the lights-out policy at McCormick Place, researchers recorded some 29,000 dead birds of 140 different species over a 25-year period.

Ken Wysocki, past president of the Chicago Ornithological Society, said almost all of the buildings in Chicago that should turn lights out during migration now do so.

Efforts of the Audubon Society and the Chicago Bird Collision Monitors were boosted by the city, which promoted a lights-out program starting about four years ago and played a key role in persuading buildings to cooperate.

"We've really made huge strides in Chicago," said Wysocki. "Ninety to ninety-five percent of buildings that should have their lights off do have their lights off."

Buildings turn out their lights between 11 p.m. and dawn, ending mass kills that sometimes resulted in janitors in the morning shoveling piles of dead birds off the sidewalks and into trash bags.

RARE BIRDS THREATENED

Particularly distressing for bird lovers is the danger posed to rare or declining species.

Among the known casualties is the Kirtland's warbler, one of America's rarest birds, whose U.S. breeding population numbers just a few hundred in northern Michigan. More common dead specimens found in New York during May included a veery, a variety of thrush and an ovenbird, a kind of warbler.

New York lags Chicago in its efforts to turn out lights, although its bird death toll has declined since the Sept. 11, 2001 destruction of the twin towers, said Rebekah Creshkoff, founder of Project Safe Flight bird-protection group.

Creshkoff urges Manhattan building managers to turn their lights out during migration but admitted it's an uphill battle in a city whose illuminated skyline is one of its defining characteristics.