The city's skyscrapers will defer to nature at least twice a year: dimming lights in September and October during the peak of the fall migratory season, and again in April and May, during the peak of the spring migratory season.

Daniel Klem, an ornithologist at Muhlenberg College in Allentown, Pa. estimates that more than 100 million birds die each year from crashing into glass on structures of all types.

And lights, particularly those from skyscrapers, distract migratory birds from the visual cues they receive from the stars and the moon, said Douglas Stotz, a conservation ecologist at the Field Museum in Chicago, especially when skies are foggy or overcast. Then the birds either crash into the building's glass at night because they are attracted to the light, or they circle the buildings until they become exhausted. In the light, they crash into the glass because of the reflection of sky.

The dead birds are swept up by custodial staff, notes Adrian Benepe, the New York City Parks Commissioner. "I've often seen them on the streets, and wondered, ???Why is this little songbird dead on the street?' "

Since 1997, Audubon Society volunteers have collected more than 4,000 dead birds of 100 different species at just a handful of buildings in Midtown and Lower Manhattan.

Toronto began a program to dim its lights in 1993, and Chicago started a voluntary program in 1999 that now includes 100 buildings. In Chicago, the Field Museum found an 80 percent reduction in bird deaths when lights were turned off during a five-year study on a single Chicago Building, McCormick Place.

Environmental groups are working with the construction industry to come up with glass that can be seen by birds, potentially by giving the glass a UV coating.

For at least 25 years, the Empire State Building building has turned off its decorative lights when large numbers of birds are observed flying around the top of the building during migration season. The circling birds are particularly common during foggy or overcast nights.