When Jon Hayes opened his email on September 9, he found a strange message waiting for him. His colleagues had been finding an oddly high number of bird carcasses, mostly of small species that migrate from the US to the tropics each fall. Two days later, he saw the birds himself while biking in Albuquerque. “I was amazed by the number of swallows that were dead at the side of the trail,” Hayes recalls. In a matter of hours, what seemed like an isolated event in southern New Mexico became national news.
Over the past week or so, the New Mexico Department of Game and Fish has received close to 300 bird carcasses, says its spokesperson Tristanna Bickford. Most of them were collected by biologists at White Sands Missile Range—where the first bodies were found on August 20—and New Mexico State University (NSMU).
“This isn’t the kind of thing that anybody who works in this field has seen before,” Hayes, a former wildlife biologist and now the executive director at Audubon New Mexico, says. “Birds that are migrating are often stressed and exhausted. But that results in a few birds here and there dying; you don’t see thousands of them dropping dead.”
Ornithologists studying the mass die-off have pointed to the wildfires on the West Coast, an extreme cold front that pushed into New Mexico last week, and a persistent drought in the area as possible explanations for what’s going on. More generally, scientists have pointed out that these extreme weather events follow predictions of how climate change alters natural cycles. “This is one more sign of a planet and ecosystem in absolute distress,” Hayes says.
Although biologists aren’t sure of the actual avian death toll, they’ve estimated a figure in the hundreds of thousands. “Just by looking at the scope of what we’re seeing, we know this is a very large event,” NMSU ornithologist Martha Desmond said in an interview with CNN.
Most of the birds have been found in New Mexico, Colorado, and along the coast of California. But the event may be more widespread: People in 12 US states and five Mexican states have reported 538 dead birds to a public database specifically created to document the die-off. But even with that data, it’s impossible to get firm numbers, especially in sparsely populated areas and wildfire-evacuation zones.
That said, every new carcass provides a body of clues for the biologists investigating the event. Most of the recovered birds are migratory insectivores like swallows, wood-pewees, flycatchers, and warblers. Most were also in rough physical shape when they died, with “no fat reserves and barely any muscle mass, almost as if they have been flying until they just couldn’t fly anymore,” NMSU’s biologist Allison Sallas shared on Twitter. The few birds that have been recovered alive are largely lethargic, Desmond said in an interview with wbur.org.
We have noticed that the majority of species collected are insectivores and long-distance migrants, such as swallows, wood-pewees, empidonax flycatchers, and warblers. (4/9) pic.twitter.com/dPoR1QFqtz
— Allison Salas (@salasphorus) September 13, 2020
Biologists suspect that the wildfires consuming the West Coast might have forced the birds to detour from their usual migration routes. While there are no major blazes in New Mexico, the state is suffering a drought, which may be depleting insect populations. “If [the birds] don’t have any food available, they will starve,” Desmond explained in the wbur.org interview.
But it isn’t only that. The smoke, which has now reached the East Coast, could be harmful to wildlife. “You taste it; you smell it; you see it every morning when you wake up,” Hayes says. “It absolutely can affect those birds as well.” Decades of research show that poor air quality can cause respiratory distress and illness in birds, affect their reproductive success, elevate their stress levels, and alter their behavior. University of Wisconsin-Madison ecologist Olivia Sanderfoot told PopSci in an email that one 2002 study “suggested that smoke inhalation may compromise the ability of birds to escape during wildfire events.”
A cold front that hit New Mexico on September 8 might have been the final straw for the flight along the West Coast. “These birds are kind of on the edge of survival during migration,” Hayes says. “When you put a 95-mile-per-hour headwind in front of them, it’s gonna exhaust them.” Typically, birds are equipped to wing through cold air currents—but this time, the rough weather might have hit them “at the perfectly wrong time.”
Of course, the string of reasons behind the mass deaths is all still hypothetical. Biologists will only be able to give hard answers after they necropsy the carcasses at the National Wildlife Health Center in Madison, Wisconsin. The analyses could take anywhere from two weeks to three months, Bickford from the New Mexico Department of Game and Fish says.
While bird die-offs do crop up now and then, they’re often linked to localized problems like pesticide spraying, new construction along migration pathways, and storm conditions. But the factors behind this calamity seem less contained. “It gets pretty scary when you think that this could become normal,” Hayes says. “Whether it’s a drought, extreme weather, or wildfires, all these [events] are likely to happen more often in a changing climate.”