manfredxy/iStock/Thinkstock(BOULDER, Colo.) — On their radar, National Weather Service meteorologists in Boulder could see it coming.

They just weren’t sure what “it” was. The mysterious red and purple blobs began appearing on radar screens, stretching at least 100 miles across the Denver area.

“The radar started to light up right after sunrise and that whole mass just started to move over Denver, all flying in the same direction, kind of floating with the light winds that day,” Paul Schlatter, a meteorologist with the National Weather Service in Boulder, told ABC News.

Thinking it might be migrating birds, the weather service tweeted out a radar animation of what they were seeing, asking bird experts for advice.

Look at what’s flying into Denver! Radar from last hour showing what we believe to be birds. Any bird experts know what kind? #ornithology pic.twitter.com/EAqzdMwpFU

— NWS Boulder (@NWSBoulder) October 3, 2017

But people on social media were seeing something else: butterflies.

Turns out they were right.

4/4: Migrating butterflies in high quantities explains it. Today, the butterflies are staying close to the ground. #cowx #Science pic.twitter.com/rkpwmPRnsi

— NWS Boulder (@NWSBoulder) October 4, 2017

“It needed to be a lot of butterflies in the area for sure to generate that much of a signal. Especially across that broad an area,” Schlatter said.

Schlatter said that while NWS meteorologists are used to seeing birds on their radar, butterflies and insects are relatively rare.

The orange-and-black butterflies, which are known as painted ladies, were passing through Colorado on their annual migration south before the winter months.

Schlatter said the flock of butterflies appeared to be at least 100 miles wide, noting that radar may not have picked up the entire mass migration because its signal eventually drops off due to the curvature of the earth.

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