The trip to McCormick Place was an unusual detour for Willard, who had stopped by on his way to work after hearing that the brightly lit building was taking out feathered flyers in droves. But in the months and years that followed, Willard found himself retracing his steps around the convention center over and over again, gathering the injured and dead.

But on an early morning in September of 1978, Willard, an ornithologist at Chicago’s Field Museum, was dismayed to find his suspicions confirmed: Speckling the pavement outside the McCormick Place convention center were the corpses of several small birds. En route to their southern homes, they’d met their ends the night before, colliding with the glass windows of the lakeside low-rise after flitting toward its unnatural glow.

It’s been more than 40 years since Willard took that fateful first walk at McCormick Place (though, on many a spring or fall morning, the now-retired ornithologist can still be spotted scouring the sidewalks near the convention center). In the decades since, Chicago’s bird body salvage program operates nearly citywide with the help of local volunteers. Together, the team has amassed tens and thousands of birds from around Chicago, representing more than 100 species.

“This work is really compelling,” says Sirena Lao , an ecologist studying bird-building collisions at Oklahoma State University who was not involved in the study. “We know from previous studies that some birds seem to be more vulnerable to these types of collisions, but it was never really clear why...and this provides insight on an aspect of bird behavior that influences [these events].”

Their work , published today in Proceedings of the Royal Society B, suggests that when it comes to bird-building collisions, light is not the only fatal attraction. Being chatty could be a migrating bird’s Achilles heel: As they careen toward buildings, social species that use chirps to navigate may continue to call to their comrades—inadvertently luring them to an untimely end.

With only corpses to query, Willard began to bring the bodies to the Field Museum. These ill-fated migrants became the first to join a vast repository that, today, numbers more than 70,000. Now, this still-growing collection is helping Willard and his colleagues uncover exactly what draws doomed birds to their deaths.

Some days, there were none. On others, after nights of buffeting winds, the bodies numbered close to 200. All, it seemed, had gravitated toward the convention center’s artificial lights. But over time, it became clear that not all birds were represented equally. Several species of sparrows, warblers, and thrushes—grimly known in birding circles as “super colliders”—were particularly abundant among the fallen.

Over the years, other ornithologists like Benjamin Winger, a biologist at the University of Michigan, began to take interest in the project. As Winger combed through the data, he was surprised to find that, despite decades of collection, warbling vireos (Vireo gilvus)—some of the most common birds on the Chicago skyline—were conspicuously absent among the fallen. Others, however, like white-throated sparrows (Zonotrichia albicollis), were oddly abundant.

Upon closer inspection, a pattern began to emerge: Among night-flying, migratory perching birds (passerines), those overrepresented in the casualty cohort—the super colliders—had something in common: They all used short vocal cues called flight calls to navigate at night. The same held true when the researchers include another set of data from Cleveland.

Flight calls are thought to help groups of birds maintain communication and travel as a cohesive unit. Like most migratory behaviors, these teeny tweets aren’t well understood, but other studies have shown call rates skyrocket when species pass over illuminated cities.

Artificial light has long been known to bewilder airborne birds, which rely on cues like starlight and a light-sensitive internal compass to maneuver through dimly lit skies. It’s thought that birds may be drawn to the glow, then get disoriented in its presence, says Sergio Cabrera-Cruz, a wildlife ecologist studying the effects of artificial light on bird migration at the University of Delaware who was not involved in the study.

Now, the new research suggests there’s more to the story than lit buildings sending birds abuzz. For some reason, birds that flight call are at a higher risk of crashing.

The researchers didn’t measure flight calls directly, so why this is remains unclear. Though these vulnerable birds have flight calling in common, the actual vocalizations might not cause collisions directly, Winger says. The culprit may be another aspect of their behavior altogether. But the possibility remains: Because vocalizations attract other birds, flight callers could be felling friends en masse, like accidental sirens beckoning sailors to their deaths.

Sparrows that died after colliding with buildings in Chicago, and were collected by researchers at the Field Museum. Image Credit: Roger Hart/University of Michigan Photography

These sobering findings underscore the need to rethink how wildlife is being affected by urban landscapes, says Rebecca Calisi Rodríguez, a biologist specializing in bird behavior at the University of California, Davis who was not involved in the study. In light-polluted cities, the same assets that help these birds make decisions as groups have now become double-edged swords, she explains.

“This really speaks to the fact that all these wonderful technologies that humans are coming up with are not always great for wildlife,” Calisi Rodríguez says.

And with humankind’s built environment ever expanding, these issues are more urgent than ever. In the United States alone, between 400 million and 1 billion birds die each year when they crash into human-made structures. (For reference, about 7.5 billion migratory birds enter the country each year.)

But as awareness grows, teams around the world are working to minimize the effects of light pollution on these avian aeronauts. In 1999, Chicago became the first city to establish a Lights Out program, encouraging building managers to turn off excess lighting during migratory seasons. As a result, deaths have dropped dramatically. Even at McCormick Place, the light regime has changed, and fatalities have decreased 75 percent since 1978. Willard’s morning strolls are now far more pleasant.

Still, the fight is far from over. Many of the most dangerous buildings for birds are those humans find most attractive or useful—towering structures that feature big windows and greenery. Reflective glass can be especially problematic for birds active during daylight hours, but they pose a hazard for nocturnal migrants as well: Birds drawn to well-lit buildings may roost near them at night, only to collide with their glossy windows the next morning.

Luckily, effective renovations are often pretty straightforward, Lao says, like the installation of bird-safe glass, which is more obviously patterned and visible, or deterrents like netting.

But there’s no substitute for simply shutting off unneeded lights—a practice that could serve up benefits to birds and humans alike, Calisi Rodríguez says. “Lights at night hurt these nocturnal species, but it’s not great for us either,” she says. “We’re animals too. Minimizing light at night…isn’t just for these birds. It would be good for all of us.”