Meteorologists in St. Louis, Missouri recently detected an unidentified and unusual object on the radar, which was moving south over southern Illinois and central Missouri.

Some people thought it was just clouds and some enthusiasts took it as a shape-shifting Unidentified Flying Object (UFO). However, the radar actually detected a large swarm of Monarch butterflies that were migrating to Mexico.

"High differential reflectivity values as well as low correlation coefficient values indicate these are most likely biological targets. High differential reflectivity indicates these are oblate targets, and low correlation coefficient means the targets are changing shape. We think these targets are Monarch butterflies," per the U.S. National Weather Service Facebook page.

Monarch butterflies (Danaus plexippus) are not endangered but their numbers are on the decline in the last few decades. The U.S. National Weather Service may have revealed the secret behind the unusual radar detection, which may have made many UFO enthusiasts unhappy but it is good news as the huge swarm of Monarch butterflies represents increasing number of the species.

The Monarch butterflies are one of the most familiar butterflies in the U.S. Its black and orange-colored wing pattern makes it very easily recognizable. They usually migrate during winters from the U.S. and Canada to Mexico in search of warmer climate.

However, climate change across the globe hovers to disturb the annual migration arrangement of the Monarch butterflies. Conservation organizations suggest that Mexico recorded the lowest number of migrating Monarch butterflies in 2013, when compared to the last 20 years' average.

Abnormal rain and drought patterns in the butterfly's breeding regions of the U.S. and Canada are believed to have caused the decline of the species. Less number of butterflies in the U.S. and Canada mean that fewer butterflies will migrate to Mexico during winters.

Monarch butterflies have been studied by many researchers. Previously, it was thought that Monarch butterflies used the Sun to navigate their long journeys. However, a previous study revealed that they have a magnetic compass of their own, which enables them to navigate even in cloudy weather in the absence of the Sun.

The Monarch butterflies are on their journey to Mexico and will return to the breeding grounds of the U.S. when spring approaches.