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Earlier this fall, when injured and dead migratory birds  on roadsides, in backyards, and along waterways across the southwestern U.S., researchers acted fast. Wildlife biologists from New Mexico State University documented the many species caught up in the event—from warblers to woodpeckers, hummingbirds to loons—and collected carcasses for further study. Biologists at the New Mexico Department of Game and Fish sent those bodies to federal laboratories, hoping for insights into the cause of death. And then, they waited.

Now, after nearly three months, shed more light on what scientists call an “unprecedented” die-off at the time. 

The findings released this month by the U.S. Geological Survey’s National Wildlife Health Center ruled out poisoning, disease, and parasites as causes of death. Instead, lab scientists found one major commonality among nearly all the dead birds: severe starvation. The carcasses shared signs of malnourishment, including empty stomachs, depleted fat stores, dehydration, and emaciation. But while it’s clear that hunger played a big part in the die-off, that's not the whole story.

One probable cause of the birds’ starvation was a severe drought in the region over the summer and into fall which made food and water scarce. “It’s been extremely dry here this year, so seed production is low and insect numbers are low,” says Martha Desmond, an avian ecologist at New Mexico State who helped to coordinate the rapid-response research effort after the die-off. Without adequate fuel, these birds were likely already in poor body condition when they arrived in the Southwest on their migrations, according to Desmond. Then came the storm.

An unseasonal cold snap rolled into the Southwest on Labor Day, and its wintry temperatures, high winds, and snow stayed in the area through most of the week. Kerry Mower, a wildlife disease specialist with New Mexico’s game and fish department, says the storm could have caused deaths in multiple ways. Some birds likely flew lower due to the storm, became disoriented, and died from striking buildings and other objects. Meanwhile, the majority of malnourished birds landed wherever they could—with deadly results. “A lot of birds in that kind of weather can’t do anything except land on the ground,” Mower says. “Many birds got caught in the snow and ice storm, and probably froze to death right there on the ground.”

While drought and the early snowstorm seem to be the major contributors to the die-off, fire might have also played a role. At the time, severe wildfires burned across the western U.S. Although lab results ruled out physical damage from smoke as a factor in most birds, Mower and others note that the fires could have contributed to some of these deaths by pushing migrants off their course and even toward the storm. “The birds could have been altering their migration path to avoid smoke plumes, thereby increasing the energy demand of their migration and causing exhaustion,” Jon Hayes, executive director of , says. “The evidence they’ve shown regarding poor body condition could still fit that scenario, and so I think there’s still questions like the role of fire in particular.”

Despite some lingering questions, Hayes and Desmond say they have no doubt that a changing climate played a part. “Nothing in the new information changes my opinion that the ultimate driver of this is changing weather patterns in the West that are easily linked to climate change,” Hayes says. Birds in the Southwest survive “on the margins,” he says, and the region’s rising temperatures, larger wildfires, and stronger storms add further stress. In other words, this fall’s gruesome mass-mortality event may have been without precedent, but it likely won’t be the last.