When Birds and Glass Collide

Annette Prince sidles along the exterior stretch of lobby windows that line a downtown Chicago office building, eyeing a tawny speck on the ground. Suddenly the speck—actually it’s a type of warbler called an ovenbird—lifts off on tiny wings, veering directly into the sheet of glass behind it. Prince makes one artful leap and gently pins the bird against the window with a small green net. Drawing it down to eye level, Prince inspects her new ward before carefully lowering it into a brown paper lunch bag on which she places a sticker with the location, species, and time of capture. “This little guy, he was pretty able-bodied as he moved around,” says Prince. But she errs on the side of caution, deciding to remove the ovenbird from the city for release somewhere safer later on.

At half-past six on this overcast morning in early May, the ovenbird’s rescue is a bright spot during a hunt punctuated by gloomier encounters. Just minutes before, Prince had scooped up a lifeless wood thrush lying in the street near the curved glass entryway of a building on the corner of Monroe and Dearborn streets. The splotch of bloody, matted feathers above its eye suggested a fatal concussion. Still earlier, around dawn, Prince had discovered a common yellowthroat next to another expanse of windows at an office building near Lake Michigan. Although motionless, the bird was still warm.

Prince has seen more than her share of avian carnage. As director of the Chicago Bird Collision Monitors, a group affiliated with Chicago Audubon, she directs a force of more than 80 volunteers who patrol the city streets during the early morning hours each spring and fall migration season, gathering victims from Chicago’s concrete canyons. Today, by morning’s end, Prince and her teammates will have rescued 17 birds and found 20 deceased, carting the living to a wildlife rehabilitation center and the rest to the Field Museum for scientific research. Prince anticipates that by the end of spring migration, she will have collected 600 injured birds and more than 700 dead ones. Those numbers, of course, don’t reflect the countless birds taken by predators, swept into the trash, or trapped out of reach.

In most cases the victims are nocturnal migrating species, such as yellow-bellied sapsuckers, northern flickers, brown creepers, hermit thrushes, and white-throated sparrows, that touch down to rest and refuel during their long journeys to wintering or breeding grounds. Urban pit stops are a mixed blessing, however. Although migrants find food and refuge in city parks and planters, the enticing vegetation, coupled with bright lights and clear glass, comprise an obstacle course that can foil even the hardiest navigators, making collisions with buildings a major cause of bird mortality. But a growing trend toward environmentally responsible building holds promise, as bird advocates, conservationists, and architects tout what they consider a vital sustainable design concept: bird safety—which, in a cruel twist, could be undermined by a building’s other environmental attributes, such as rooftop gardens and energy-efficient windows with reflective coatings. “[Architects and their clients] can use all the recycled material they want, they can save all the energy they want,” says Daniel Klem Jr., an ornithologist who has devoted his career to studying bird collisions. “But if their building is still killing birds, it’s not green to me.”

Bird strikes occur year-round and can happen at virtually any type of building—commercial, educational, or residential. But when it comes to cities that, like Chicago, lie on avian flyways, the problem is especially evident during migration season. In the evening, bright lighting on skyscrapers can lure birds in search of navigational cues typically afforded by the moon and stars. The effect is most pronounced on evenings of bad weather, when the cloud cover is low and birds are forced to fly at lower altitudes. Confused by the artificial light beams, some migrants crash into the buildings’ facades.

Most migrants, however, will settle into the city unscathed until morning, only to face a more menacing danger: glass. “It’s not, in my view, beacons that are attracting these animals to their deaths on the tops of the buildings,” says Klem, who is based at the Acopian Center for Ornithology in the Department of Biology at Muhlenberg College in Allentown, Pennsylvania. “It’s a secondary effect that gets them, and that’s the glass on the ground.”

Klem estimates that at least a billion birds—roughly five percent of the bird population after breeding season—die annually across the United States by colliding with windows, making it the second-largest manmade threat to birds after habitat loss. When confronted with a pane, most migratory species are vulnerable, because birds don’t perceive glass as a barrier. (Resident birds like pigeons seem more immune, likely because they’ve become accustomed to their environment.) In other words, glass is an indiscriminate killer, culling the healthiest members of a population as well as the weakest. To make matters worse, many of the victims are songbirds whose populations are already in decline, like the wood thrush Prince found that early May morning.