Wonders of Nature
Monarch Butterfly Sanctuary in El Rosario, Michoacan, Mexico
Those Butterflies migrate to Mexico every year from Canada and Northern parts of US. They travel over 4000km one way and can cover as much as 100km a day. They are the only Butterflies that make both, north and south migration on a regular basis; however, it is not the same individuals that go back. It is the third or fourth generation that returns to its northern locations, which they leave because of cold temperatures and lack of food in winter months. 
They always go back to exactly the same locations, how they do it it's still a mystery. 

one sat on Marta's hat

the branches are orange because of the butterflies sitting on them

when is cloudy they don't fly, but gather together on trees; those black 'cones' are branches hanging down because of the weight of butterflies

couple of videos of what words cannot describe