Millions of monarch butterflies migrate across the North American continent each year, traveling thousands of miles at a time.

Millions of monarch butterflies migrate across the North American continent each year, traveling thousands of miles to warmer environments.

This week, National Weather Service radar picked up a cloud of monarchs flying over the St. Louis, Missouri area.

They weren’t sure what the radar was picking up at first because of the fluctuating shape.

The cloud of butterflies spread over 250 miles into Illinois and Missouri, as the colorful insects made their way south for the winter.

The National Weather Service St. Louis posted on their Facebook page: "A Monarch in flight would look oblate to the radar, and flapping wings would account for the changing shape! NWS St. Louis wishes good luck and a safe journey to these amazing little creatures on their long journey south!”

The populations of monarch butterflies have been drastically reduced since the 1990s, and experts say that habitat loss from illegal logging in Mexico, along with the use of herbicides by farmers in the United States have both led to the insects decline.

Conservationists in the city of St. Louis have encouraged the planting of milkweed, which is the only source of food for monarch caterpillars.