Thanks to a bit of help from bird enthusiasts and on-the-ground locals, a cloud-like pattern recently seen moving across the US state of Colorado has been identified as a 70 mile-wide swarm of butterflies.

Image: National Weather Service/Facebook

Meteorologists with the National Weather Service (NWS) first noticed the sizeable mass on their Doppler radar earlier this month, but misidentified it as a flock of birds. In the hopes of finding out which species of feathered friends they might have encountered, the NWS team posted a video of the discovery on social media. It didn't take long, however, for ornithologists to chime in and poke a hole in the hypothesis: the cloud was moving in the wrong direction.

Winter has already begun to take its icy toll on Colorado, sending local birds southwards in search of warmer weather. The cloud on the radar was headed north. An alternative explanation came when Denver residents began reporting that thousands of painted lady butterflies (from the subgenus Cynthia) had suddenly descended on their gardens and homes.

Tiny animals like butterflies rarely produce radar signatures of this clarity and magnitude, but when enough insects link up, it's possible for them to be confused for something much larger.

"Things with big wings need to fly together in the same direction with the wind to generate that signature," NWS wrote on Facebook. "Migrating butterflies in high quantities explains it!"

The butterflies appear to have taken off in El Paso, Elbert, and Douglas counties before finally forming an impressive kaleidoscope that drifted across Denver. It's hard to say what prompted the rapid jump from city to city, but the day after NWS detected the high-activity signature, the winged travellers who caused it appeared to be laying low, spending their time gulping down nectar.

Butterfly numbers tend to increase when flowers are plentiful, so it's possible that recent weather changes sent the population towards more promising food sources. NWS meteorologists suspect cold snaps in the area may have had something to do with the rapid migration picked up by their instruments.  

"We cannot recall seeing this radar signature in our area before," the team said. "There was something unique about Tuesday ... apparently the butterflies felt compelled to migrate northwest."

Some butterflies actually "surf" wind currents to save energy during their lengthy journeys, so it's also possible that the shift towards Denver was incidental – the butterflies may have simply been blown off course. That might explain why even people living on Colorado's snow-dusted mountain ranges reported hundreds of butterflies around their alpine homes during the event.

"I couldn’t believe my eyes," local resident Susan Schinner wrote on Facebook in response to the NWS video. "We live at 10,000 ft. and we have about four inches of snow on the ground. When I took the dogs for a walk, there were butterflies everywhere!"

In any case, these animals aren't likely to continue on a northerly trajectory. Colorado-based lepidopterist Sarah Garrett explains that painted ladies tend to move towards warmer habitat in places like Arizona, New Mexico, and even the upper reaches of Mexico as temperatures drop. In fact, last month a different painted lady population crossed through Colorado's Front Range on their way further south.

"I have been getting phone calls from people all over the Front Range in many different counties,” Garrett told The Denver Post at the time. "Last week, I spoke to folks in North Dakota and South Dakota who have seen them. They are making their way progressively through these Western states."

Where will the orange visitors head next? Help scientists find out out by reporting your sightings!



Top header image: coloneljohnbritt/Flickr