The real story
Each autumn eastern monarchs fly up to 3,000 miles from Canada and the US to a few mountain forests in central Mexico, where millions cluster on fir trees so thickly the branches bend. This 'super generation' lives about eight months. The summer generations live only a few weeks.
Navigating with a sun compass and an internal clock, they find forests they've never seen. In spring they head north and lay eggs, and it takes three or four generations to reach Canada again. The ones that fly south next autumn are the great-great-grandchildren of the last ones who did.