It is one of the Earth's great migrations: each year, millions of monarch butterflies fly some 3,000 miles, from their summer breeding grounds as far north as Canada to their overwintering sites in the central Mexico. It's one of the best-studied migrations and in recent years, ecologists like Sonia Altizer have been able to better answer how and why these intrepid butterflies make the journey. Short Wave brings this episode from the TED Radio Hour's episode with Sonia Altizer, with the University of Georgia.