There are so many butterflies in Denver that they showed up on radar

The great mass of colors spread all across Denver and neighboring counties, and the weather scientists weren’t sure what to make of it.

A radar image that likely shows butterflies. (National Weather Service)
A radar image that likely shows butterflies. (National Weather Service)
A radar image that likely shows butterflies. (National Weather Service)

The National Weather Service posted an image of a bewildering blob this week. The great mass of colors spread all across Denver and neighboring counties, and the weather scientists weren’t sure what to make of it.

Their best theory on Tuesday was that they were looking at birds, which made me want to hide out inside for a few hours. Today, though, they think they have a better answer: migrating butterflies.

The folks in the Boulder meteorology office originally had ruled out insects, since they “rarely produce such a coherent radar signature.”

But they realized later, with the help of social media users, that a loosely spaced group of insects with big wings actually could show up if they were all flying in the same direction.

Butterflies loitering in the Golden Triangle neighborhood. (Kendall Smith/Denverite)
Butterflies loitering in the Golden Triangle neighborhood. (Kendall Smith/Denverite)

“Migrating butterflies in high quantities explains it,” they posted. And we sure do have a lot of those right now.

It’s migration season for the painted lady butterfly, the orange-and-black beauties that are making the journey right now from north to south. Their populations travel between the southwestern United States/northern Mexico and the central U.S., according to The Prairie Ecologist.

The butterflies appeared to cover an area about 70 miles wide and to float along winds to the northwest on Tuesday. That was another big clue, as migratory birds would likely have been headed straight south. Butterflies can take a less direct route, following the winds.

The painted ladies have descended on Denver in the last few weeks in unusually large numbers, feeding on flowers and flying in groups.

Sarah Garrett, a lepidopterist at the Butterfly Pavilion in Westminster, Colorado, said people from as far away as the Dakotas have called to report seeing the butterflies, whose population typically surges with plentiful flowers.

Research on painted ladies in North America is limited, but scientists believe they migrate to the southwestern United States and northwestern Mexico in the fall. In Europe, studies using radio tracking have shown they migrate south from Europe to Africa in the fall and return in the spring.

Studies also show that the larger monarch butterflies often use wind to their advantage and glide on currents for periods of time as they migrate, Garrett said. The longest single-day flight by a monarch was longer than 250 miles, according to the U.S. Forest Service.

The Associated Press contributed to this report.

Andrew Kenney

Author: Andrew Kenney

Andrew Kenney writes about public spaces, Denver phenomena and whatever else. He previously worked for six years as a reporter at The News & Observer in Raleigh, N.C. His most prized possession is his collection of bizarre voicemail. Leave him one at 303-502-2803, or email akenney@denverite.com.