“It’s a bird, it’s a plane, it’s… a flock of migrating butterflies!” After spotting a colored mass flitting over Denver and nearby counties, weather scientists at the National Weather Service supposed the phenomenon was just a group of birds. With the help of social media users, however, they later realized that the immense cloud was comprised thousands of butterflies. It turns out, there are so many butterflies migrating across the central U.S. that they showed up on the radar.
— NWS Boulder (@NWSBoulder) October 3, 2017
Weather scientists at the Boulder meteorology office posted the images to social media with the caption, “Look at what’s flying into Denver! Radar from last hour showing what we believe to be birds. Any bird experts know what kind?” After confirming that avians “rarely produce such a coherent radar signature” and taking into account social media users’ answers, the Boulder meteorology office realized they were actually butterflies.
“Migrating butterflies in high quantities explains it,” the group posted afterward. The Denverite reports that it is presently migration season for the painted lady butterfly. Orange-and-black in color, the butterflies are making their way from north to south, in time with the changing seasons. According to The Prairie Ecologies, thousands of the painted ladies butterflies travel between the southwest part of the United States/northern Mexico and the central U.S. every year.
Because butterflies migrate with the wind, they were able to cover an area about 70-miles-wide. Birds, on the other hand, fly straight toward their destination. This was a big clue in differentiating the mass of flying objects. Said Sarah Garrett, a lepidopterist at the Butterfly Pavilion in Westminster, Colorado, people as far away as North and South Dakota have spotted the butterflies, whose populations typically surge when flowers are abundant.
Scientists believe the painted lady butterflies migrate to the southwestern United States and northern Mexico in the fall. Using radio tracking, studies have shown they also travel south from Europe to Africa in the fall, and return in the spring.