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Migrating birds are dying in staggering and disturbing numbers throughout Southwest Colorado, and researchers are at odds about the reasons why.

Reports of widespread bird mortality began in the past week or so, said Colorado Parks and Wildlife spokesman Joe Lewandowski, who is based in Durango.

“We’re getting a lot of calls,” he said. “And it’s fairly widespread.”

David Porter, who lives in the Animas Valley, said he’s found six dead birds on his property in just the past five days. Oddly, he said, the birds, typically smaller songbirds, show no signs of injury.

“And that’s just the ones I’ve seen in obvious locations,” he said. “There’s no telling if I was to walk the whole perimeter if there’s more. Who knows?”

Lisa Wilk said her husband recently visited Pastorius Reservoir, a few miles southeast of Durango, where it appeared someone had piled up about a dozen dead swallows.

“It’s really tragic,” she said. “I don’t know what’s going on. I know people are researching it, and making a lot of guesses.”

A recent report in the Las Cruces Sun-News, which has garnered widespread attention, said that over the past few weeks an “unprecedented” amount of birds have been found dead from unknown causes in southern New Mexico.

It appears, speaking with local researchers in Durango and several residents, the mysterious deaths of numerous birds are happening here, too.

The first thing to note is how the birds are being found dead, said Catherine Ortega, an ornithologist and ecological consultant based in Durango. Typically, she said, birds hide under a bush or in a shrub to die.

“It’s very strange when you get birds just dropping from the sky,” she said. “You just don’t see them dead out in the open. So when you do, you can tell something different is going on.”

It’s all speculation at this point, Ortega said, but she believes the birds found dead in Southwest Colorado are actually birds from the West Coast, pushed out and stressed by the unprecedented amount of wildfires.

“Maybe they’re being forced to migrate early, and they’re not ready to leave,” she said. “And so if they’re escaping, and their lungs have been damaged by the smoke, maybe that’s what’s making them drop.”

A recent cold snap in Southwest Colorado last week brought snowfall and a dramatic dip in temperatures in the high country, which may have caused certain species to also start their migration earlier than normal.

Joe Szuszwalak, a spokesman for the U.S. Fish and Wildlife Service, said the cold front disrupted migration routes, causing birds to drop into areas in the southern U.S. where water and food supplies to regain energy are in short supply.

“This is a natural event that has been known to occur periodically,” he said. “We are working with our state and federal partners, including the USGS National Wildlife Health Center, to further research this event.”

But Arvind Panjabi with the Bird Conservancy of the Rockies said birds are hardy and can typically withstand even early season winter storms. Instead, the cold snap could have acted as a one-two punch with the wildfire smoke.

“It’s possible they were seeking refuge and may have been weakened when the storm hit,” he said. “It’s really hard to say without definitively determining the cause of death for these birds.”

It’s unclear if there’s any active study of the dead birds found in Southwest Colorado.

The Las Cruces Sun-News, however, reported bird carcasses found in southern New Mexico were sent to the Fish and Wildlife Service’s Forensics Laboratory in Oregon, though it could take weeks before results are back in.

“It is terribly frightening,” Martha Desmond, a professor at New Mexico State University, told the Sun-News. “We’ve never see anything like this ... We’re losing probably hundreds of thousands, if not millions, of migratory birds.”

An estimated 28 different wildfires have consumed more than 3.2 million acres in California this year, and 1 million acres have burned in Oregon, sending dense smoke throughout the western U.S.

An August 2017 study published by the Institute of Physics highlighted just how sensitive birds’ lungs are to air pollution, including wildfire smoke, which can cause respiratory distress and illness, elevated stress levels and behavior changes.

“A fire in April of 1999 caused the death of 50 adult white ibises (Eudocimus albus) found on a cattail island,” the study said. “The ibises likely became trapped due to the presence of thick smoke.”

Panjabi said the wildfires are sending tons of pollutants into the sky at a time when birds are at peak migration.

“These massive fires are inescapable,” he said. “And there is ample evidence birds are very sensitive to airborne toxins. That’s why they put canaries in coal mines, they die more rapidly when exposed to toxins.”

In the western part of the U.S., birds typically follow two migration paths: the Pacific route, extending from the West Coast to about the Continental Divide, and the central route, encompassing the east side of the divide through the Midwest.

Southwest Colorado falls right in between the two flyways.

“I think we’re seeing (dead) birds from the Pacific flyway that have shifted to the central,” Ortega said. “So we’re not losing the entire country’s birds ... and hopefully, populations recover in a few years, but that remains to be seen.”

When surmising about the recent die-off of birds, Ortega said she’s keeping in mind the scientific principle of Occam’s razor, which says when confronted with a complicated issue, usually, the answer is the most simple explanation.

For her, that means the culprit is wildfires, which can have a cascading effect – forcing birds into early migration, sometimes on unfamiliar routes, wiping out food sources and damaging lungs.

“All the fire and smoke, it’s a strong candidate for the explanation,” she said. “But people are scrambling to figure this out.”

The public is encouraged to report mortality events to the USGS at https://on.doi.gov/3kozIoS.

jromeo@durangoherald.com