Swarm of migrating butterflies shows up on weather radar

Oct. 6 (UPI) -- The National Weather Service said a huge mass seen descending on Denver in weather radar imagery was found to be a gigantic swarm of butterflies.

The NWS station in Boulder posted an animated GIF image to Twitter showing a radar image of something they initially hypothesized to be migrating birds.

"Look at what's flying into Denver! Radar from last hour showing what we believe to be birds. Any bird experts know what kind?" the NWS tweeted Tuesday.

Bird experts responded to the tweet, saying the image was taken at an odd time of day to be birds, which generally migrate at night.

The NWS initially brushed off suggestions that the image could be a swarm of insects, but it was later noted that the mass was moving north-northwest rather than south, as it would be if it was a flock of birds.

Twitter users explained the butterflies were attempting to migrate south, but were being blown off-course by the wind.

The station explained why the image was originally thought to be birds.

"Things with big wings need to fly together in the same direction with the wind to generate that signature in ZDR," NWS wrote on Facebook. "Insects rarely produce such a coherent radar signature. Migrating birds do all the time."