Monarch butterflies really are beautiful and cool in many ways. One ability which is helpful in their travels is to use the earth's magnetic field for navigation. This comes in handy on cloudy days and their schedule still calls for some serious mileage north or south. For all practical purposes they have a little compass built in somewhere in their bodies.

Ref: J.A. Etheredge et al (1999) "Monarch butterflies ( Danaus plexippus L.) use a magnetic compass for navigation" Proceedings of the National Academy of Sciences, vol 96, no 24, pp 13845-6.