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Every spring, as birds fly north to their breeding grounds, they run a gauntlet unique to the modern era: forests of towering buildings in urban centers from coast to coast. Hundreds of millions crash into structures and fall to their deaths—some attracted and confused by buildings lit up at night and others simply not seeing windows and glass in day. 

Now has unearthed an underlying pattern to the ecological calamity. Migrating song birds that vocalize, or call, at night during their flights are far more likely to strike buildings than those that are silent.

The study, published in the journal Proceedings of the Royal Society B, looked at nearly 40 years of data from Chicago, where dead birds were collected at the foot of buildings with lights on at night. It compared the rate of fatalities among different bird species to a smaller data set in Cleveland, where a community group, Lights Out Cleveland, also monitored buildings.

The trend was the same in each city: Warblers, thrushes, sparrows and other species that call during nocturnal migration suffered far more fatalities than did species such as quiet vireos, gnatcatchers, and flycatchers.

Scientists believe that migrating birds use flight calls while on the move at night to aid in navigation and tweak their trajectories. When buildings glow at night in urban centers, one bird might vocalize to others with a message that, roughly translated, means, “Hey, check it out!”

Study author Benjamin M. Winger, an assistant professor of ecology and evolutionary biology at the University of Michigan, says that although the birds don’t necessarily migrate in tight flocks, their calls nonetheless allow them to “maintain some cohesion.” When they become disoriented by artificial light, he says, “they are calling a lot,” drawing others near. That, the study notes, may spawn a “a vicious cycle of increased mortality rates.”

To reach this conclusion, researchers from the University of Michigan, the Cornell Lab of Ornithology, the Cleveland Museum of Natural History the Field Museum in Chicago analyzed an unusually large data set: 69,785 collision records involving 93 species that migrate mostly at night. About half came from a single building, McCormick Place, a lakefront convention center that is a lethal magnet due to its recessed windows and history of illumination. Between 1978 and 2016, Field Museum staff would walk around the facility during migration seasons, gathering bird casualties and sealing them in plastic bags. Other records came from , a volunteer network that began similar work in 2002. In Cleveland, the study considered 2,229 records of building strikes involving 62 species.

Some species flew to their deaths so frequently that the researchers dubbed them “super colliders.” They included White-throated Sparrows, Song Sparrows, Dark-eyed Juncos, and Ovenbirds, all of which vocalize in transit at night. For example, in Chicago, researchers collected more than 10,000 White-throated Sparrow victims over the study period. By contrast, they found only two Warbling Vireos and six Blue-gray Gnatcatchers during the same period, despite their abundance—all three species are in the top 25 most reported eBird species in Chicago.

Susan B. Elbin, director of conservation and science at New York City ���Ϲ���, said the findings suggest the importance of , which urge building owners to sign a pledge to dim lights between midnight and dawn during spring and fall migrations. In New York, about 100 buildings have joined so far.

“It’s really fascinating and important for us to look at the whole light landscape to see how that affects bird behavior during migration,” she says. But Elbin thought it would be too complicated for building managers to time their lights-out schedules to the arrival of the most vulnerable bird species.

“It’s easier just to get people to respect the two migrations instead of trying to tweak it and figure out who is coming through,” she says. “Maybe as the science gets a little more predictive, it might make sense.”

From Portland, Oregon to Baltimore, Maryland, voluntary Lights Out programs are growing in popularity. But city governments are also instituting both guidelines or rules to encourage bird-friendly facades. In San Francisco, an ordinance established bird-safe standards for new buildings and retrofits, insisting on glass treated so that birds can detect it or decorated with dots or decals—that mostly helps day-flying birds. But light is also part of the ordinance. Owners must avoid up-lighting and event search lights and reduce external lighting. In Chicago, careful long-term cataloguing of building strikes has helped the push for change, resulting in a building code that now requires many new large buildings to control and reduce lighting. However, the study authors caution much work remains.

In Cleveland, which has fewer skyscrapers, ornithologist and study author Andrew W. Jones hopes to convince at least a few dozen of the tallest buildings in the downtown to join a two-year-old Lights Out initiative there, partly to set an example. “That could seed a community-level awareness, with more buildings throughout all of Cleveland and its suburbs turning off their lights at night,” he says. “It's really not just a tall-building issue.”