Birds carry out some of the most amazing feats of athleticism in the world. Hummingbirds cross the entire Gulf of Mexico, their tiny wings beating continuously for three days straight. A single bird will fly across the entire Pacific ocean in one go. What do we really know about bird migration, and how do we know it? This week we're talking with Rebecca Heisman about her new book: Flight Paths: How a Passionate and Quirky Group of Pioneering Scientists Solved the Mystery of Bird Migration.