The Overnight Roost

Monarch butterflies only migrate during the day. At night, they come down to rest. They spend the night in trees, clustered together in “roosts” or “bivouacs.”

In the morning, the butterflies must warm up before they can fly. To fly WELL they need to attain thoracic temperatures at least 55 F (13 C). (Monarchs cannot even crawl when their thoracic temperatures are below 41 F (5C).)

As this map shows, temperatures drop as the fall season progresses.

Monarchs have a shorter and shorter window of time each day in which they can fly.

This is one reason monarchs must leave the north before it gets too cold!


Watch how the temperatures fall in the fall!

Try This! Too Cold to Fly Today?

  • Record and graph the daily temperatures in your hometown this fall. When does the temperature drop below 55F?
  • As the season continues, for how many hours in each 24 hour day is the temperature below 55F?
  • Note that monarchs can bask in the sun and/or shiver to raise their temperatures above air temperature. Therefore, keep track of sunny and cloudy days to get a better sense of when monarchs could fly.

National Science Education Standards

  • Weather changes from day to day and over the seasons.
  • The behavior of individual organisms is influenced by internal cues (such as hunger) and by external cues (such as a change in the environment).
  • Regulation of an organism's internal environment involves sensing the internal environment and changing physiological activities to keep conditions within range required to survive.