This article originally appeared in longer form in bioGraphic. 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. And, as they linger and forage in Chicago’s urban canyons, they collide with glass. 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 …