Every year, millions of Monarchs descend on just a few different sites on the California coast and the mountains of Mexico to overwinter. There are so many, they literally cover the trees and ground, as the pictures below show. Scientist are learning more about how Monarchs find their way to the overwinter location. From Wikipedia:
"How the species manages to return to the same overwintering spots over a gap of several generations is still a subject of research; the flight patterns appear to be inherited, based on a combination of the position of the sun in the sky and a time-compensated Sun compass that depends upon a circadian clock that is based in their antennae. New research has also shown that Monarch butterflies can use the earth's magnetic field for orientation. The antennae contain cryptochrome a photoreceptor protein that is sensitive to the violet-blue part of the spectrum. In the presence of violet or blue light it can function as a chemical compass, which tells the animal if it is aligned with the earths magnetic field, but it is unable to tell the difference between the magnetic North or South. The complete magnetical sense is present in a single antenna."Chemical compass? Now that is awesome!