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A dozen dead Barn and Violet-green Swallows huddled together on the dusty desert floor of southern New Mexico. Numerous Western Bluebirds packed into a crevice in southern Colorado as if they panicked. Sparrows, lined up almost wing-to-wing, lying limply along the banks of the Rio Grande.

These are just a few of the grisly discoveries recently made in what is likely a mass death event for migratory birds occurring across the Southwest. At the moment, there is no clear explanation. 

The die-off is “unprecedented,” says , an avian ecologist at New Mexico State University (NMSU) in Las Cruces, who is leading the research team documenting the event. She estimates that hundreds of thousands and possibly even up to a million birds have died across at least five U.S. states and in four Mexican states. “It’s enormous, the extent of this,” Desmond says. “We haven’t counted all the species yet, but there are lots of species involved.” Online reports show dead owls, warblers, hummingbirds, loons, flycatchers, woodpeckers, and more—representing the  heading south to their wintering grounds.

The exact reasons for the deaths aren’t yet known. A cold snap that brought snow, wind, and low temperatures on September 8 and 9 could have forced birds to migrate early or brought down birds already weak from migration. Similarly, wildfires raging along the West Coast might have spurred premature departures while also interfering with birds' migratory routes, vision, and breathing. Some combination of both factors may also be the cause, but experts emphasize that nothing has been proven so far. “There’s more questions than answers still,” says Jon Hayes, executive director of .

Scientists first began reporting avian deaths throughout New Mexico in August. Initially they didn’t think anything particularly unusual was going on: Birds expend a massive amount of energy flying hundreds or thousands of miles while also dodging deadly threats like bad weather, predators, and buildings. “The tragic but true fact of migrations is that birds die,” Hayes says. “Migration is very tough.”

But as reports of bird deaths became more widespread and continued into September, researchers started to become alarmed. More and more photos showing dead and weak birds on the ground were posted to a regional listserv, and observations of abnormal behavior, atypical flight patterns, and stray or vagrant birds across the Southwest further supported some sort of mass catastrophe.  

With the situation growing more dire, the NMSU scientists sprang into action. Desmond quickly convened wildlife experts from the university, the Bureau of Land Management, and , where a large number of birds were found dead on August 20. Since then, the collaborative research team has already begun a sweeping study of as many migratory birds as they can collect, living or dead, to understand what might have happened. Along with examining bird carcasses—more than 300 so far—researchers are catching and banding migrants passing through. 

The first possible cause the researchers considered was recent unseasonal weather in the Southwest, which brought temperatures in the 30s and 40s, high winds, and snow to parts of the region. “A lot of birds probably died with the weather event that happened a week ago,” Desmond says. It’s also possible the cold spell forced birds to depart on their migration earlier than anticipated, she says. But the storms abated last week and birds continue to die. “It’s also very troubling that all of this started well before the [cold] weather, and it’s still continuing after the weather.”

The ongoing wildfires in California, Oregon, and Washington could also be playing a role. Wildfires are known to force early migration movements from bird species, and the smoke can poison the air while decreasing visibility. “The wildfire smoke is significant . . . You couldn’t see across the street,” Hayes says, regarding air quality conditions from his home in Placitas, New Mexico. “There’s no doubt in my mind that’s going to affect birds, too.”

Hayes sees a connection between these different extreme weather events. “This is about abrupt changes in our weather patterns as a result of climate change,” he says. “All these things are going to cause long-term declines, long-term losses [of birds], and they’re gonna be punctuated by big scary events like this. It’s part of this bigger problem.”

A 2019 study led by the Cornell Lab of Ornithology found that North America is currently home to 3 billion fewer birds than it was 50 years ago due to multifold changes to habitat and food sources. Also last year, scientists with the Լ����Ƶ used 140 million bird sightings to project in the coming century. They found that 389 bird species, including some killed in the current die-off, are threatened with extinction as temperatures and rainfall patterns shift. Many are also at risk from weather events like wildfires made more extreme by the warming climate.

I just recorded this up in Velarde, N.M. I've never seen anything like it. I'm told of other dead migratory birds found in Hernandez, Ojo Sarco and El Valle de Arroyo Seco.

— objectivity haver (@austieJFish)

So far, the most detailed information about the bird die-off has come from New Mexico, but after NMSU staff built a , numerous reports of related migrant bird deaths have now come from five states in the southwestern United States and four states in Mexico.

After their initial investigation, the researchers plan to send the hundreds of avian carcasses they've collected to the in Ashland, Oregon, which could potentially provide more key information through necropsy reports. The results could take week or more, and even then, clear answers may remain elusive.  “Until we really get these birds out to autopsy, we won’t know,” she says. “It’s possible the autopsies won’t even tell us what went on.”

In the meantime, researchers are encouraging birders in the Southwest to be on the lookout for disoriented or dead birds, and to report any observations and photos to iNaturalist or state fish and game departments. The U.S Fish and Wildlife Service is also collecting carcasses of birds affected, and instructions for sending them can be found .