When a flock of starlings encounters a predator, they swarm in what’s called a murmuration. CC0 Creative Commons

A few hundred birds mysteriously fell out of the sky outside of Salt Lake City this week, spooking residents and sparking conspiracy theories about aliens.

So many starlings dropped to the ground Jan. 29 in Draper, a city just south of Salt Lake City, that a man was using a shovel to clear birds out of the roadway, according to KSTU, a Fox affiliate. The city’s animal services department found 17 birds still alive and is caring for them until they can be released back into the wild.

“As I was driving, these birds were just falling out of the sky,” one resident, Lacey Brown, told KSTU. “They were all on the ground right around here and on the roadway. … They were just falling out of the sky like leaves.”

Before the birds dropped, they were exhibiting a defensive behavior called murmuring, in which the small birds swarm together in a massive flock that looks like a black cloud in the sky.

“The beauty of a murmuration’s movements often arises purely out of defense, as the starlings strive to put distance between themselves and the predator,” the Cornell Lab of Ornithology explains. “When one starling changes direction or speed, each of the other birds in the flock responds to the change, and they do so nearly simultaneously regardless of the size of the flock.”

Falcons are an example of a predator that could cause starlings to flock in this way.

The Draper City Police Department suggested that a bunch of the starlings in the cloud over their city crashed into something, causing the falling birds, according to KSTU. That’s despite some talk on social media of aliens or a lightning strike.

“No aliens, no cloaking device,” Sgt. Chad Carpenter told KSTU, “No poison, it was just one of those freak things where the birds were just flying along, crashed into the side of a large vehicle and boom.”