A new report based on necropsies conducted by the USGS’s National Wildlife Health Center in Wisconsin suggests that the die-off was caused by long-term starvation and was likely exacerbated by a spate of freakishly cold weather, reports Phoebe Weston for the Guardian. Estimates suggest hundreds of thousands of birds may have died and nearly 10,000 dead birds were reported to a wildlife mortality database by citizen scientists, per the Guardian.
Scientists arrived at starvation as the most likely cause of death in the majority of the birds they examined based on depleted fat deposits, empty stomachs, small amounts of blood, kidney failure and shrunken flying muscles, reports the Associated Press. Many birds also had irritated lung tissues.
While the findings didn’t identify a single, direct cause of death, they ruled out poisoning, disease and parasites as potential culprits, reports Kevin Johnson for Audubon.
“It looks like the immediate cause of death in these birds was emaciation as a result of starvation,” Jonathan Sleeman, director of the USGS National Wildlife Health Center where the necropsies were conducted, tells the Guardian. “It’s really hard to attribute direct causation, but given the close correlation of the weather event with the death of these birds, we think that either the weather event forced these birds to migrate prior to being ready, or maybe impacted their access to food sources during their migration.”
Martha Desmond, an ecologist at New Mexico State University, tells Algernon D’Ammassa of the Las Cruces Sun News that she expects similar findings to emerge from a second analysis being conducted by the U.S. Fish and Wildlife Service, which she says "should be released soon."
Migrating birds arrived in New Mexico in “poor body condition” with some birds already beginning to starve, according to a statement from the New Mexico Department of Game and Fish. "The unusual winter storm exacerbated conditions, likely causing birds to become disoriented and fly into objects and buildings,” the statement continues. “Some were struck by vehicles and many landed on the ground where cold temperatures, ice, snow and predators killed them."
Desmond tells Audubon that the birds may have arrived in the southwestern U.S. already starving in part because of severe drought afflicting the region.
“It’s been extremely dry here this year, so seed production is low and insect numbers are low,” says Desmond, who helped organize research efforts to study the die-off. With less food, the birds would have lacked the stores of energy needed to complete their grueling migrations.
Though the first reports of dead birds started coming in on August 20, an unseasonably cold storm blew in around Labor Day and coincided with the largest number of observed deaths, per the Guardian. Beginning on September 9, temperatures in Albuquerque plummeted from a high of 97 degrees to lows of 39 degrees, wrote John C. Mittermeier in an October blog post for the American Bird Conservancy. According to the Albuquerque Journal, the cold snap broke a 105-year-old record for the lowest temperatures seen at that time of year and was accompanied by high winds that toppled power lines in the area.
The frigid conditions and high winds sent large numbers of birds flying lower in search of some respite, according to Audubon. Some may have become disoriented, leading to deadly collisions with buildings and cars, but most were simply forced to land. “A lot of birds in that kind of weather can’t do anything except land on the ground,” Kerry Mower, a scientist with New Mexico Game and Fish, tells Audubon. “Many birds got caught in the snow and ice storm, and probably froze to death right there on the ground.”
According to the Guardian, researchers and members of the public raised concerns over the last few months speculating that the fires in California might have re-routed birds through the Chihuahuan desert and contributed to the die-off. However, the report did not find any signs of smoke damage in the lungs of the examined birds.
Jon Hayes, executive director of Audubon Southwest, tells Audubon he thinks the birds may have re-routed before incurring serious lung damage.
Ultimately, Hayes and Desmond say drought and the bout of unseasonal, extreme weather point to one ultimate cause of the die-off: climate change.