In the beginning there was great confusion about animal migration. Aristotle, noting that the types of birds around him changed with the seasons, concluded that summer redstarts turned into robins at the onset of winter, and that garden warblers became blackcaps [1]. Others thought that birds disappear in winter because they hibernate submerged in mud. In a case of art decidedly not imitating life, a 16th century illustration accompanying the writings of Swedish Archbishop Olaus Magnus showed a fishing net filled with hibernating swallows being pulled from a lake [1]. Gradually, over centuries, these fanciful early explanations gave way to an understanding that migration is a widespread phenomenon and that Earth is alive with itinerant animals traversing continents, seas, and skies (Figure 1).