The weather was beautiful in St. Louis, Mo. on Friday morning: 70 degrees and clear blue sky. Which is why forecasters were surprised by what they were seeing on their radar screens, which they now think were migrating Monarch butterflies.

“It was a completely clear day — there were not even high clouds — and so we were all kind of scratching our heads saying ‘what are we looking at here?'” said Laura Kanofsky, a meteorologist for the National Weather Service in St. Louis. “It was not ground clutter, or mid-level clouds, which we can sometimes get in Friday’s conditions.”

Radar is sometimes a finicky technology. It’s rooted in simple physics: send out radio waves in all directions and wait for them to bounce back. Usually, the things that radio waves bounce off of are rain drops. But often the radar can pick up other objects in the sky, like dirt and dust picked up by the gust front of a squall line, or migrating birds, or even bats as they leave their caves for their evening hunts.

But this time it was not bats, or birds, or debris. Weather Service meteorologists are keenly familiar with the reflectivity from birds on radar. But this signature was different.

“High differential reflectivity values as well as low correlation coefficient values indicate these are most likely biological targets,” explained the NWS St. Louis office on Facebook. “High differential reflectivity indicates these are oblate targets, and low correlation coefficient means the targets are changing shape.”

Honing in on insects, meteorologists turned to the internet to figure out what might be migrating this time of year. Sure enough, Monarchs had been spotted to the north of the forecast area. While forecasters aren’t totally sure what it was, it’s likely the radar caught the butterflies mid-flight on their way south to Mexico where the species spends the winter. “A Monarch in flight would look oblate to the radar, and flapping wings would account for the changing shape!” wrote the forecast office.

From the radar echoes, it might seem as if there was quite a swarm of butterflies heading south on Friday. However, it doesn’t take much for the radar to light up when it’s dry out. “In dry conditions, the radar is very sensitive to something like insects,” said Kanofsky. “It doesn’t take a whole lot of insects to create a high return.”

As a meteorologist, you don’t picture yourself spending time digging for information on migrating bugs on the internet, trying to figure out why the radar is glowing when there’s not a cloud in the sky. But maybe that’s what makes the job so much fun. “It was not something that we were expecting,” said Kanofsky. “It was really interesting.”