What’s Causing the Mass Bird Die-Off in the Southwest?

This story originally appeared in The Guardian and is part of the Climate Desk collaboration.

Thousands of migrating birds have inexplicably died in the southwestern US, in what ornithologists have described as a national tragedy that is likely related to the climate crisis.

Flycatchers, swallows, and warblers are among the species “falling out of the sky” as part of a mass die-off across New Mexico, Colorado, Texas, Arizona, and farther north into Nebraska, with growing concerns that there could be hundreds of thousands dead already, said Martha Desmond, a professor in the Biology Department at New Mexico State University (NMSU). Many carcasses have little remaining fat reserves or muscle mass, with some birds appearing to have nose-dived into the ground in mid-flight.
“I collected over a dozen in just a 2-mile stretch in front of my house,” said Desmond. “To see this and to be picking up these carcasses and realizing how widespread this is, is personally devastating. To see this many individuals and species dying is a national tragedy.”Long-distance migrants flying south from tundra landscapes in Alaska and Canada pass over America’s southwest to reach winter grounds in Central and South America. During this migration it is crucial that they land every few days to refuel before continuing their journey.
Historic wildfires across the western United States could mean they had to reroute their migration away from resource-rich coastal areas and move inland over the Chihuahuan desert, where food and water are scarce, essentially meaning they starved to death. “They’re literally just feathers and bones,” Allison Salas, a graduate student at NMSU who has been collecting carcasses, wrote in a Twitter thread about the die-off. “Almost as if they have been flying until they just couldn’t fly any more.”

The southwestern states of the US have experienced extremely dry conditions—believed to be related to the climate crisis—meaning there could be fewer insects, the main food source for migrating birds. A cold snap locally between September 9 and 10 could have also worsened conditions for the birds.

Any of these weather events may have triggered birds to start their migration early, having not built up sufficient fat reserves. Another theory is that the smoke from the wildfires may have damaged their lungs. “It could be a combination of things. It could be something that’s still completely unknown to us,” said Salas.

“The fact that we’re finding hundreds of these birds dying, just kind of falling out of the sky, is extremely alarming. The volume of carcasses that we have found has literally given me chills.”

The first deaths were reported on August 20 on White Sands Missile Range in New Mexico. Initially, incidents were thought to be unrelated, but thanks to online forums, ornithologists noticed that they were happening all across the region. Resident bird species, such as curve-billed thrashers, great-tailed grackles, and white-winged doves do not appear to have been affected.

Reports suggest some birds have been displaying unusual behavior before dying—becoming lethargic and approachable and congregating in groups. Species that normally rest in trees and shrubs have been seen hopping around on the ground looking for insects, said Desmond.

Large avian mortalities during migration are rare, and few have been as large as this one. Records—which go back to the 1800s—show that these events are always associated with extreme weather events such as a drop in temperature, snowstorm, or hailstorm. The largest event on record in the region was a snowstorm in Minnesota and Iowa in March 1904 that killed 1.5 million birds.The climate crisis is also changing the tundra landscape where many of these birds breed, while the destruction of rain forests in Central and South America is damaging their winter habitats. Since 1970, 3 billion birds have been lost in the US and Canada. Mass die-offs such as this can have an effect on populations of both common and sensitive species. Salas says, “We’re kind of coming at them from all sides. If we don’t do anything to protect their habitat, we’re going to lose large numbers of the populations of several species.”