Mystery of millions of migrating birds dropping dead from US skies

Experts are struggling to explain a mystery mass die-off of migratory birds in the southwestern US that biologists have described as “devastating”.

Flycatchers, swallows and warblers are among the species “falling out of the sky” across New Mexico, Colorado, Texas, Arizona and as far north as Nebraska, according to Martha Desmond, a professor at New Mexico State University (NMSU).

Scientists began investigating the phenomenon in late August after ”a large number of dead birds” were found at the US army’s White Sands Missile Range and White Sands National Monument in New Mexico, CNN reports.

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“We know this is a very large event, hundreds of thousands and maybe even millions of dead birds, and we’re looking at the higher end of that,” Desmond told the US news channel.

The biologist says that a number of the species are “already experiencing huge population declines, and then to have a traumatic event like this is devastating”, reports New Mexico-based newspaper Les Crucas Sun News.

According to witnesses, “some birds have been displaying unusual behaviour before dying”, with species normally seen in shrubs and trees spotted on the ground looking for food and chasing insects, says The Guardian.

Some biologists have suggested that the wildfires raging across the West Coast of the US could be to blame. One theory is that the smoke has affected the birds’ migration patterns, forcing them to fly inland over the Chihuahuan desert, where there is little food or water.

“They’re literally just feathers and bones,” said Allison Salas, a graduate student at NMSU who has been collecting carcasses. “Almost as if they have been flying until they just couldn’t fly anymore.”

Other possible explanations put forward include a recent “cold snap” in the Mountain States that could have worsened conditions for the birds, and droughts that have depleted insect populations in the southwest.

Any of these weather events “may have triggered birds to start their migration early, having not built up sufficient fat reserves”, according to The Guardian.

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.

As efforts to unravel the mystery continue, scientists are urging people living in the affected areas to log any sightings of dead birds on the iNaturalist social network.