Strange Cloud Turns Out to Be Butterflies Migrating

by Stephen Luntz

Photo credit: NWS St Louis. Butterflies migrating formed the shape of a butterfly on radar.

One of the world's great migrations has returned. In a cute twist, its shape at the largest scale seems to reflect that of the creatures involved.

The decline of the monarch butterfly (Danaus plexippus) has come to symbolize the destruction of nature. Hundreds of millions of Eastern American monarchs travel up to 5000km in their journey from their feeding grounds in across much of North America to winter in an astonishingly small area of Mexico, although according to one theory the extent of the migration is a relatively recent response to deforestation.

Once the migration was a wonder of nature for numbers as well as distance. However, the destruction of their main food source has sent numbers into sharp decline, with additional damage done by deforestation and climate change. By 2013 the crash was estimated to have slashed numbers by 90%. 

So when the National Weather Service (NWS) picked up a large cloud on their St Louis radar on a sunny day, their first thought was not that it was made up of millions of butterflies. On further investigation however, the service announced the way in which the radar was being reflected indicated the signals sources were “most likely biological targets”. Moreover, the shape and changes in the signal suggested butterflies flapping their wings.

The suspicion was confirmed when the NWS St Louis Facebook page returned numerous accounts of monarch sightings just to the north shortly beforehand. The observations had an extra touch of synchronicity with the shape of the cloud resembling a butterfly.

The radar images are not enough on their own to conclude the monarchs are bouncing back. The NWS's Laura Kanofsky cautions, “In dry conditions, the radar is very sensitive to something like insects. It doesn’t take a whole lot of insects to create a high return.” However, early reports from Mexico suggest that butterfly arrivals are at least doing better than last year's record low, if nothing like returning to pre-1990s levels.

Monarch's beauty comes from a color scheme designed to warn predators of an unpleasant taste from the presence of toxic cardiac glycosides. Nevertheless, monarchs are an important food source for many birds. While individual monarchs make the southward trip, the spring migration north is conducted by three generations, each only surviving for a few weeks and making only part of the species odyssey, providing plenty of food for those who prey on them.

The giant cloud comes only a month after the filing of a petition to preserve the monarch under the Endangered Species Act citing drastic declines in estimations of monarch numbers over the last two decades.