This video from the USA says about itself:
Pacific monarchs migrate 2,500 miles between California and Mexico. This 10 minute segment captures some of the thousands of butterflies along the journey.
From Wildlife Extra:
Inbuilt compasses help monarch butterflies migrate
How new generations of monarch butterflies, despite never have travelled the distance before, find their way from their breeding sites in eastern United States to their overwintering habitat in central Mexico has long puzzled scientists.
Previous studies have revealed that the butterflies use a time-compensated sun compass in their antenna to help them make their 2,000 mile migratory journey to overwintering sites.
However how they found their way under dense cloud cover remained a mystery.
US scientists, using flight simulators equipped with artificial magnetic fields, found that if they changed the fields the monarchs oriented in the opposite direction, to the north instead of the south.
“Our study shows that monarchs use a sophisticated magnetic inclination compass system for navigation similar to that used by much larger-brained migratory vertebrates such as birds and sea turtles, ” said co-author Robert Gegear, from Worcester Polytechnic Institute.
“For migratory monarchs, the inclination compass may serve as an important back up system when daylight cues are unavailable.
“It may also augment hand-in-hand with the time-compensated sun compass to provide orientation and directionality throughout the migration process.”
To work, the compass is light dependant, relying on a certain wavelength of ultra-violet ray that can penetrate dense cloud.
However this study also opens up the possibility that the monarch survival could be vulnerable to potential disruption of the magnetic field.
“Greater knowledge of the mechanisms underlying the autumn migration may well aid in its preservation, currently threatened by climate change and by the continuing loss of milkweed and overwintering habitats,” said senior study author Steven Reppert of UMass Medical School.
“A new vulnerability to now consider is the potential disruption of the magnetic compass in the monarchs by human-induced electromagnetic noise, which can also affect geomagnetic orientation in migratory birds.”