Alien spaceship? Toxic fumes? 5G technology? These were some of the theories flighted online in response to surveillance footage showing hundred of birds mysteriously plummeting from the sky in the northern Mexican city of Cuauhtémoc. A short clip recorded on a security camera shows a large flock of yellow-headed blackbirds plunging onto a suburban street like a haze of black smog. Several of the birds are left strewn across the road while the majority of the flock flies off.

According to local reports the "bird rain" took place on February 7 and initial insights from a local veterinarian suggested the cause of the strange behaviour could be linked to high levels of pollution in the area as a result of wood-burning heaters, agrochemicals and cold weather. Electrocution caused by collisions with powerlines was also put forward as a possible reason for the plummet, while online audiences offered more creative explanations including 5G technology and invisible spaceships.

Although the incident is certainly unusual and the circumstances are ripe for speculation, ornithologists and bird experts have since weighed in with a more ordinary explanation. "This looks like a raptor like a peregrine or hawk has been chasing a flock, like they do with murmurating starlings, and they have crashed as the flock was forced low,” Dr Richard Broughton, an ecologist with the UK Centre for Ecology & Hydrology, explained to The Guardian. Some bird species such as starlings, blackbirds and queleas form tight-knit flocks when travelling to aid in deterring predators that may struggle to pick out an individual target. Yellow-headed blackbirds congregate in large migratory flocks as they flee the chilly winter weather in Canada and the northern United States and head to Mexico in search of warmer climes. It's likely that this particular group were targeted by a dive-bombing predator and some of the birds made a fatal miscalculation while trying to escape.

"This truly was an ‘oops’ moment for the birds ... A really big ‘oops’ moment," Kevin J. McGowan, an ornithologist at the Cornell Lab of Ornithology, quipped to The Washington Post.

"The big flocks can make these quick twisting motions, move together very quickly and they all are following the one right next to them who makes the most decisive move," McGowan said. "They’re wingtip to wingtip in this tight bunch that makes it harder for the predators to actually pick out one and keep up with it." In a tightly packed flock, the birds follow the movement of the one in front of them and aren't always able to assess their broader surroundings. Occasionally, this results in collisions with the ground or other infrastructure.

"Birds are light and fluffy, so they have a lot of wind resistance," McGowan explained. "They don’t fall straight down very quickly, like a baseball. And in this case, they went flying down. It was a purposeful motion."

It's unclear how often this occurs, but there are other records of mass bird die-offs that have been attributed to a moment of poor judgement on the birds' part. In 2019, over 200 deceased starlings were found on a road in Wales. Despite a slew of Hitchcockian, paranormal theories doing the rounds, investigators concluded that the birds died after striking the tarmac possibly to escape a raptor or severe weather in the area.