Everyone has heard the thunk of a bird hitting a window. It’s startling, and then it’s sad. But what few of us realize is just how widespread the problem is. Across North America, buildings account for hundreds of millions of bird deaths annually—perhaps more, according to the nonprofit group the American Bird Conservancy. A running estimate of North America’s collision-killed birds—posted online by another nonprofit organization, Toronto’s Fatal Light Awareness Program—ticks along at roughly 30 bird deaths per second. After habitat destruction, collisions with buildings are the single biggest killer of birds.