Scientists and forecasters were mystified last week when a strange butterfly-shaped cloud appeared over the city of St. Louis, MO last week. Even more amazing was what the cloud turned out to be: an actual swarm of monarch butterflies.

Last Friday afternoon, an enormous swarm of migrating monarch butterflies formed a butterfly-shaped cloud that appeared on local radar between 5,000 and 6,000 feet up in the sky. It is suspected that the butterflies, which would have numbered in the hundreds, were migrating south to Mexico.

The cloud appeared to be flapping, indicating a biological origin as opposed to a technological creation. It was originally thought to be a group of hummingbirds, but was too high up- hummingbirds usually fly just slightly above the tree level.

This isn’t the first time that a radar image has strangely mirrored a natural phenomenon. A flock of blackbirds in Arkansas in 2011 looked like a bird’s beak and head.

According to the nonprofit Monarch Watch, the butterflies have begun migrating from the Great Lakes region down to central Mexico, where they will spend the winter in the Michoacan region. There is also a California population that spends its summers in the state, but also migrates to Mexico when the temperatures begin to drop.

It’s likely that the monarchs formed a swarm to take advantage of wind currents. While sightings of a few monarchs traveling together are quite usual, it’s strange to see a swarm numbering in the hundreds. They typically use favorable wind speeds to conserve energy during their two-month migration to Mexico.

The monarch butterfly population has experienced a decline in recent years, owing to pesticides and droughts along their migration route and in the western central Mexican region. Only 33 million monarch butterflies spent the winter in Mexico last year, setting a record low for the population.