The massive death of thousands of songbirds in the southwestern United States was caused by long-term starvation, compounded by unusually cold weather, likely related to the climate crisis, the scientists said.
Flycatchers, swallows and warblers were among the migratory birds that “fell from the sky” in September, with carcasses found in New Mexico, Colorado, Texas, Arizona and Nebraska. A necropsy by the USGS National Wildlife Health Center found that 80% of the samples showed typical signs of starvation.
The muscles that control the birds’ wings were severely shrunken, blood was found in their intestinal tract, and they had kidney failure, as well as a general loss of body fat. The remaining 20% were not in sufficient condition to perform the proper tests. Nearly 10,000 dead birds were reported by citizens to the wildlife mortality database, and previous estimates suggest that hundreds of thousands may have died.
“It appears that the immediate cause of death in these birds was emaciation as a result of hunger,” said Jonathan Sleeman, director of the USGS National Wildlife Health Center in Madison, Wisconsin, who received 170 bird carcasses and performed necropsies on 40 of them. “It is really difficult to attribute direct causation, but given the close correlation of the meteorological event with the death of these birds, we believe that the meteorological event forced these birds to migrate before they were ready or perhaps affected their access to food sources during their migration “.
The first deaths were reported Aug. 20 at the White Sands Missile Range in New Mexico, with accounts of birds appearing lethargic and congregating in groups before dying. Most of the deaths occurred between September 9 and 10 during a cold season that likely meant that food was particularly scarce. In a weakened state, the birds appeared to become disoriented, flying towards cars and buildings, some dying from the impact and others landing on the ground, subsequently dying from cold temperatures or being eaten by predators.
“We’re not talking about short-term starvation, it’s longer-term starvation,” said Martha Desmond, a professor in the department of biology at New Mexico State University (NMSU), who collected dead bodies. “They got so emaciated that they actually had to dedicate themselves to losing their main flight muscles. This means that this is not something that happened overnight. “
The birds probably began their migration in poor conditions, which could be related to the “mega-drought” in the southwest of the country. “Here in New Mexico we have seen a very dry year and it is forecast that we will have more of those dry years. And, in turn, I would say that it appears that a change in the weather is playing a role in this, and that we can expect to see more of this in the future, “said Desmond.
“I think it’s very sad,” he added. “Especially the idea that we are seeing some long-term starvation in some of these birds.” Sleeman could not say whether this event was directly related to climate change, but acknowledged that the likelihood of extreme weather events is increasing.
Desmond previously described watching so many individuals and species die as a national tragedy. Most of the birds were insect and berry eaters that migrated from tundra landscapes in Alaska and Canada to winter in Central and South America. Most of them have to stop and refuel every few days during their migration.
Concerns were raised about the wildfires in California that caused the birds to change their migration route inland over the Chihuahuan desert, but necropsies showed no smoke damage to their lungs. The center also tested for contagious bacterial and viral diseases, parasites and evidence of pesticide poisoning, all of which came back negative.
Allison Salas, a graduate student at NMSU, said the volume of corpses she had collected had given her chills. She said: “The fact that we find hundreds of these birds dying, simply falling from the sky is extremely alarming.”
Desmond’s team hopes to get funding to support more investigations into the mass bird kills so they can better monitor what is happening. Sleeman agreed that large-scale wildlife mortality events are occurring more frequently. “It’s something we definitely need to keep track of,” he added.
By Phoebe Weston. Article in English