Radar maps from St. Louis, Missouri, showed something strange last week: a large, slow-moving cloud that was changing shapes as it drifted south. And one of those shapes, captured above in a screen shot by the National Weather Service, coincidentally offered a clue about the cloud's identity.

After analyzing the mysterious reflections, meteorologists determined this wasn't a thunderstorm or a radar glitch. It was most likely monarch butterflies, flying by the millions from Canada to Mexico in one of North America's most impressive — and also endangered — annual migrations.

"Keen observers of our radar data probably noticed some fairly high returns moving south over southern Illinois and central Missouri," the NWS says. "We think these targets are Monarch butterflies. A Monarch in flight would look oblate to the radar, and flapping wings would account for the changing shape!"

High differential reflectivity in the radar images suggests the objects are oblate, the NWS explains, while a low correlation coefficient suggests the individual targets pictured are changing shapes. Taken together, this points to a biological storm rather than a meteorological one, the agency concludes. It would also match recent ground observations of monarch butterflies flying through the region.

monarch butterfly

Monarch butterflies as a species are not endangered, but their North American migration might be. (Photo: William Warby/Flickr)

That's a welcome sight across North America, since the continent's iconic monarch butterfly migration fell to its lowest numbers on record in 2013. It has been in decline for several years, mainly due to the loss of milkweed, a vital food source for monarch caterpillars. Milkweed is dwindling across North America amid the widespread use of broad-spectrum herbicides, but it's not the only factor in the butterflies' decline. They've also lost swaths of important habitat to deforestation, and they're increasingly vulnerable to severe weather and temperature changes brought by climate change.

There may be good news on the horizon, though. While it's too early to draw conclusions, experts in Mexico say monarchs are entering the country earlier than usual this year. Only time will tell if this means a rebound in the butterflies' numbers, but researchers at least have cause for optimism.

In the meantime, meteorologists in the U.S. Midwest took time to cheer on the insects' improbable adventure, which spans three countries, 2,500 miles and four generations of butterflies: "NWS St. Louis wishes good luck and a safe journey to these amazing little creatures on their long journey south!"

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