Murder hornets, zombie cicadas, invasive lizards -- the list of unexpected creatures popping up across the United States appears to be growing this year.

Next on the list? Massive bat colonies, just in time for Halloween.

Meteorologists at the National Weather Service (NWS) in Phoenix were looking at their radar screens and spotted a large cloud of motion sweeping over areas of the city last Sunday, just a week before the official start of fall.

"That doesn't look like a normal shower, the way everything is sort of fanning out," NWS meteorologist Sean Benedict told CNN affiliate KTVK. "They don't really have a uniform direction. That's usually your clue initially that it's probably animals flying around."

The team quickly surmised that they were looking at an enormous colony of bats.

They were spotted around sunset, suggesting that the critters are Mexican free-tailed bats that migrated to the city for the summer, Arizona Game and Fish Department biologist Angie McIntire told KTVK.

Radars readings like the one posted on the official NWS Twitter account, which is believed to show the bats coming out of a cave, tunnel or bridge, help scientists track the nocturnal animals, which are difficult to spot in the dark.

Although the flying mammals are often associated with spooky vampires, they actually snack on mosquitoes and are quite harmless.

"Bats get a really bad rap," McIntire said. "But there's really nothing to fear when it comes to bats."

There are 28 known bat species living in Arizona, where residents often go bat watching. While the best time to catch them is between July and August when the colony reaches their peak, they can also be seen as late as October.

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