After a “mass collision event” last year where thousands of disoriented birds crashed into city buildings and died, nearly twenty high-rise buildings in Philadelphia are dimming their lights at night to reduce such incidents in a “Bird Safe Philly” campaign, CNN reports.

While people might think that brighter lights help birds miss buildings, the opposite is true.  Birds and other animals navigate using the sun, moon and bright stars as navigation points.  Bright lights in multiple buildings at night, in overcast skies or in foggy conditions can disorient birds and cause single birds or flocks to fly directly into buildings, killing them.

“Suddenly they have all these lights coming at them from different directions. It’s overwhelming,” Robert Peck, a senior fellow at the Academy of Natural Sciences of Drexel University, said. “They get turned around and they will fly into buildings and walls.”

In October 2020, thousands of birds flew into buildings, littering sidewalks below with their carcasses.  “The ground was sprinkled with dead and dying birds,” Peck said.

The effort to dim city lights parallels efforts to reduce light pollution from cities in the US and around the world.  More than one billion birds die every year from building strikes.

The first recorded “window strike” in Philadelphia happened in the 1890s, when the tallest building in the city was Philadelphia City Hall.  For nearly a century, until 1987, city ordinance mandated that no building in Philadelphia could be higher than William Penn’s hat on City Hall’s tower, 548 feet above the ground, high above the nine floors of the building’s office space.