The 2020 wildfire season caused a mass die-off of birds in the western and central United States, according to crowdsourced science and weather location data summarized in a new study in the journal GeoHealth.

“After an abnormally large number of migratory birds turned up dead in people’s backyards in Colorado and other parts of western and central U.S. states, locals began to document their observations on a crowdsourced science platform called iNaturalist,” Wildfire Today reports. The GeoHealth study went a step further, combining the iNaturalist data—with recordings of migratory birds like warblers, geese, hummingbirds, swallows, flycatchers, and sparrows—with map readings that matched the birds’ deaths with locations that had seen wildfires or snowstorms.

There was a lot of that activity to document. “Around the same period as the birds’ deaths, more than three million hectares (7.8 million acres) of land burned, which resulted in habitat loss and the emission of toxic compounds that threaten the health of both avian species and humans,” Wildfire Today says. Some parts of the northwestern U.S. were also hit with early September snowstorms while birds were in the midst of their annual migration, with temperatures in some areas dropping by as much as 40°C/72°F within hours.

“The wildfire and also the toxic air were the two factors that influenced the birds’ mortality,” said study co-author Anni Yang, a postdoctoral fellow in spatial ecology at Colorado State University, pointing to a strong correlation between the bird deaths, wildfires, and toxic gases. The study found no similar connection to winter storms, though Yang said that might have been a matter of fewer people making observations outdoors during bad weather.

Wildfire Today says birds’ respiratory systems are easily damaged by air pollution, but today’s combination of stresses is a new challenge. “Although wildfires have always occurred and birds have evolved to cope with them in some measure, the combination of climate change and decades of fire suppression in parts of the United States has led to fires that burn far hotter and larger than fires that burned in centuries past,” the publication notes. 

The study found larger numbers of bird deaths farther from wildfires in some areas, suggesting that some of them may have been killed by unusually hot, humid air over oceans. The data also pointed to land cover as a factor, with more birds dying in urban areas, though Yang said pandemic quarantines may have biased that result.

“People were spending more time in urban areas and perhaps paying more attention to dead birds around their homes,” Wildfire Today explains, “which inadvertently may have boosted the numbers in cities compared with rural areas.”