A colourful, shimmering spectacle detected by satellites over the US state of Colorado has been identified as swarms of migrating butterflies.

Scientists at the National Weather Service (NWS) first mistook the orange radar blob for birds and had asked the public to help identifying the species.

They later established that the 70-mile wide (110km) mass was a kaleidoscope of Painted Lady butterflies.

Forecasters say it is uncommon for flying insects to be detected by radar.

“We hadn’t seen a signature like that in a while,” said NWS meteorologist Paul Schlatter, who first spotted the radar blip.