In the beginning of October this year, staff at the Boulder meteorology office in Denver, Colorado witnessed something rather strange on their radar screens. It initially seemed as though a huge group of birds- about 70 miles across- was flying into the city. The staff then ruled out the possibility of an insect horde too, since they “rarely produce such a coherent radar signature.”
Finally, the scientists got their answer: it was an enormous group of painted lady butterflies, slowly making their way from north down to warmer climes in the south.
“Migrating butterflies in high quantities explains it,” they said, after coming to the conclusion that the painted lady butterflies, which travel between the central and southwestern United States and northern Mexico every autumn, showed up on the radar due to their large wings and because they were all flying in the same direction.