Because butterflies are poikilotherms, meaning they adjust their body temperature to the ambient air temperature, they can remain inactive, without worrying about freezing to death, and yet maintain the energy they'll need for their northward journey. When the days lengthen, the monarchs descend from the trees and begin to mate before flying north, usually during the last two weeks in March. They flutter along at speeds of generally between 5 to 20 miles per hour, but may attain speeds of 30 miles per hour. The trip north is more of a solo effort than is the fall migration. Females lay their eggs in the southern U. S. and then die, but their offspring continue northward.
Evidence indicates that they fly only during the daylight hours, resting in trees along the way.
Some observers have noticed that many kinds of migrating butterflies attempt to fly in a straight line and will go over a tall obstacle in its path instead of flying around it, even it going around it would be the easier solution. Migrating butterflies have been known to fly through railway tunnels rather than changing their course. Actually, relatively little is known about the altitudes at which they fly, if they utilize magnetic sensing as do many birds, or if they set their sights on the Sun and use it as a compass. The big swarms have been seen flying from a few feet off the ground to hundreds of feet in the air. It makes sense that they would want to fly high enough off the ground to avoid having to go over trees and buildings.