Hundreds of thousands, perhaps more than a million migratory birds died across the Southwest and Rocky Mountains suddenly late in the week of Sept. 7.
Arroyos and hillsides across New Mexico were littered with corpses, causing alarm among both scientists and the general public. Suggested causes varied from wildfire smoke inhalation to pesticides to a new avian infection.
But a team of scientists at the University of New Mexico has good reason to think the cause was more mundane: the cold snap that occurred earlier that week, when temperatures went from 93 degrees to 33 degrees in less than 48 hours.
Jenna McCullough is a third-year PhD student at UNM and the Museum of Southwestern Biology studying birds in the South Pacific. She is also a third-generation birder, sharing a love of the animals with her mother and grandmother.
“Every ornithologist loves birds so deeply,” McCullough said. “And the public loves birds too. They’re very charismatic. And you can find birds wherever you go in the world, which is why I love birds so much.”
When she saw a video on social media of hundreds of dead birds near Velarde, she and fellow PhD student Nick Vinciguerra drove north from Albuquerque at 8 pm to collect specimens for research and preservation. When they arrived two hours later, birds of several species, mainly violet-green swallows with some Wilson’s warblers and other swallow species, lay scattered on the ground.
“I felt like I was on the front lines of a mass extinction,” she said. “It really struck both of us really deep, you know, we collected birds in silence. And the only words we were saying was, ‘Oh my God, Oh my God, I’m so upset right now.’ I was kind of choking back tears.”
The two collected several gallon-sized plastic bags full of bodies, something that other scientists across the state and region were also doing. McCullough and Vinciguerra weren’t back home until 4 am.
Kathleen Ramsay is a veterinarian at Cottonwood Veterinary Clinic, in Sombrillo, and founder of the New Mexico Wildlife Center in Española, which routinely deals with injured wild birds. Calls started coming in, and she began collecting bodies.
“We had birds coming from Amalia, and El Rito birds, birds from Taos, birds from Velarde,” she said. “We were just a dropoff space, so that Game and Fish didn’t have to drive all over New Mexico to pick up the bodies to send them in.”
She said she’s never seen anything like this, but drew a comparison to when West Nile Virus appeared about 20 years ago and killed off magpies, crows and jays around her house. But the scale then was much smaller.
“Everybody wants to blame it on smoke,” she said. “Everybody wants to blame it on fire retardant. We have no clue. Let’s find out what it is and then put that out to people.”
Birds collected by Ramsay and other scientists were organized by the state Department of Game and Fish and shipped to federal laboratories in Oregon and Wisconsin for thorough pathology reports. Those lab results are still pending as of press time and will provide more conclusive evidence toward the birds’ cause of death, whether caused by smoke, insecticide, infection or exposure.
However, McCullough saw obvious evidence that the cold snap was to blame for at least the proximate cause of death, although many questions remain unanswered.
When temperatures drop suddenly, insects die or become dormant and birds no longer have a source of food. Additionally, this time of year is when birds need calories the most, to fuel their thousand-mile migrations across the continent.
The lack of food, cold weather, wet conditions and extremely high winds on top of regular migration stress overtook many birds not just in New Mexico, but across the Southwest and Rocky Mountain region. McCullough points to similar documented die-offs in Kazakhstan in 2000 and Central Europe in 1931 and 1974.
Museum of Southwestern Biology Director Chris Witt suggested McCullough examine and weigh the birds and compare that data to historical examples. Because the museum kept their specimens on-hand, she was able to take measurements right away.
“Chris was like, ‘well, you know, if you think it’s due to the cold snap, you should go and weigh those birds and compare them,’” McCullough said.
An online database of biological measurements called Vertnet enabled her to look at the average weight of violet-green swallows collected across a similar time period. That data shows the average weight of a normal bird to be about 15 grams; the average bird collected at Velarde weighed just 9.5 grams, which indicates that most birds had little to no fat on their bodies. This was supported by visual observations.
McCullough shared her findings on the American Birding Association’s website Sept. 18. While not conclusive, she’s convinced it’s the most likely explanation.
“I think it will be really interesting to see what the autopsies come back as, but I, if I was a betting woman, would put money down on the starvation hypothesis,” she said.
While the explanation has precedent in history and doesn’t indicate a coming apocalypse of novel cause, the influence of climate change and wildfires in the West are important parts of the questions that remain. Birds may have taken more easterly migration routes to avoid fires and smoke along the West Coast, and thus were caught in already unfamiliar territory during the record-breaking cold snap.
These snaps are likely to become more common in the future.
“We know that extreme weather events are occurring more frequently because of climate change, climate change changes precipitation patterns,” McCullough said. “It changes weather patterns over the long term. You can’t point to one hurricane or one event and say, climate change, but you can point to decades, a century of these things changing and say climate change.”
“The point of my article was that this was more of a naturally occurring thing, but I’m not meaning to say that you shouldn’t take climate change seriously,” she said. “We should take it seriously, but we can’t just point and wave our arms and say it was wildfires.”
If people want to protect birds, the biggest thing they can do as individuals is keep their cats indoors, according to McCullough.
“Cats are predators, they’re invasive and they are a huge problem to birds,” she said. “It’s not a fun thing to say, but feral cats are a huge problem.”
She pointed to research from the University of Georgia indicating that cats, both domesticated and feral, potentially kill up to 500 million birds per year nationwide.