Every spring, as the daylight lengthens and the weather warms, rivers of birds flow north across the Midwest. They fly high and at night, navigating via the stars and their own internal compasses: kinglets and creepers, woodpeckers and warblers, sparrows and shrikes.
They come from as far as Central America, bound for Minnesotan wetlands, Canadian boreal forests, and Arctic tundra. They migrate over towns and prairies and cornfields; they soar over the black tongue of Lake Michigan in such dense aggregations that they register on radar. Upon crossing the water, many encounter Chicago, where they alight in whatever greenery they can find—office parks and rooftop shrubs and scraggly street-trees and the sparse landscaping outside apartment-complex lobbies.
To us humans, glass is ubiquitous and banal; to birds, it’s one of the world’s most confounding materials. A tanager or flicker flying toward a transparent window perceives only the space and objects beyond, not the invisible forcefield in its way. The reflective glass that coats many modern skyscrapers is just as dangerous, a shimmering mirror of clouds and trees. Some birds survive collisions, dazed but unharmed. Most don’t, done in by brain injuries and internal bleeding. Per one 2014 analysis, glass kills as many as a billion birds every year in the United States alone.