A butterfly swarm over the US state of Colorado was so huge that it was picked up by a satellite weather radar.
Scientists at the National Weather Service (NWS) first mistook the colourful, shimmering spectacle for a flock of birds and had asked for the public to help to identify the species.
But they later discovered the 70-mile wide mass was a kaleidoscope of migrating Painted Lady butterflies.
It is uncommon for flying butterflies or insects to be detected by a satellite radar, according to experts.
NWS meteorologist Paul Schlatter, who first spotted the orange radar blob, told CBS News: “We detect migrating birds all the time but they were flying north to south.”
He said the direction of travel would be unusual for migratory birds for the time of year.
Mr Schlatter asked the NWS’s Twitter followers for help to determining the species of what he believed were birds.
He later wrote: “Migrating butterflies in high qualities explains it.”
Painted Lady butterflies, which are about three inches long, have descended on the Denver area over the last few weeks.
The species travel to northern Mexico and the US southwest from other parts of the country during colder months.