Philadelphia has dimmed its skyline after a 'mass collision' killed thousands of migrating birds

Nearly twenty buildings in Philadelphia are dimming their lights this spring after thousands of birds perished in the city's largest mass collision in recent history.

The voluntary measure has been coordinated by "Bird Safe Philly" and continues until May 30 with the purpose of sparing the lives of some of the 100 million birds passing over the City of Brotherly Love on their semiannual migrations. To some, it might seem more intuitive to leave the lights on to help birds steer clear of buildings, but Robert Peck, a senior fellow at the Academy of Natural Sciences of Drexel University, explains most migrating fowl navigate by light; the sun, the moon and the stars.