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A swarm of hundreds of birds plummeted to the ground outside your house. Carcasses lay strewed in the street, the birds killed from the impact.
That’s the view a handful of people woke up to in Chihuahua, Mexico, on February 7.
Millions around the world got a front-row seat of the massacre when security camera footage was released of the event, but it left people with more questions than answers.
Why did the birds crash?
“The cause of this bizarre and troubling incident is honestly anybody’s guess at this point,” said Carlos Botero, assistant professor of biology at Washington University in St. Louis.
The footage didn’t show the entire swarm of birds, but Botero hypothesized that they could have flown through a cloud of lethal chemicals. Autopsies of the dead bird specimens would need to be completed to determine if that was the case, he said.
A predator could have also sent the birds frantically flying away, Botero added.
Richard Broughton, an ornithologist at the UK Centre for Ecology & Hydrology, was almost certain the maneuver was to avoid a predator like a peregrine falcon.
“Blackbirds form tight flocks, called a ‘murmuration,’ that swirls in the sky to try and confuse the falcon so it cannot pick a target,” Broughton said via email.
To combat this strategy, the falcon dives straight into the flock of birds to separate out a target, Broughton explained. When this happens, the blackbirds try to avoid it.
In the video, viewers are likely seeing the birds try and escape a predator that attacked them from above, he said. The birds headed down, but some could not pull up fast enough, Broughton added.
Assuming the time stamp of 7:42 p.m. local time on the video is accurate, the birds were likely leaving their nocturnal roost nearby, said Andrew Farnsworth, senior research associate at The Cornell Lab of Ornithology in Ithaca, New York.
In addition to the predatory theory, Farnsworth suggested that a loud noise startling the birds was another possibility. This has happened in the past with other species of birds, he said.
How common is this behavior?
It’s very rare for birds to crash into the ground, and it’s not normal behavior, Botero said.
There are other occurrences of birds dying from abrupt crashing, Farnsworth said.
In 1999, about 110 king eiders, a type of large sea duck, were found dead on Baffin Island in Canada. There weren’t any obstacles in their way, so it’s likely they crashed because of poor visibility, which could be attributed to the cataracts in their eyes, a study found.
In 1985 and 2003, dozens of geese were found dead in fields in southern Manitoba, Canada. Researchers initially thought they were poisoned, but the severe injuries the geese sustained hinted that they could have become disoriented on the moonless nights or been frightened by a thunderstorm.
What are these birds?
Experts identified the birds as yellow-headed blackbirds, migratory birds living in the western and prairie wetlands of North America. In the winter they form large flocks, which is what was caught on video.
The eBird, an online database of bird observations, at The Cornell Lab of Ornithology tracks these birds’ migration pattern on a map as they fly south to Mexico for the winter then back to the United States and Canada for the summer. At the time the footage was recorded, most the birds would have been in Mexico with a few scattered in the Southwestern United States.