As winter approaches, V-shaped flocks glide overhead as the world's birds begin their long treks to warmer climates. Humans used to have some pretty crazy theories about where birds went for winter, like the moon, or to the bottom of the ocean. How did we learn the real story? And where DO birds go for winter? What are the longest bird migrations? How do birds store up energy for their long journey? How do birds navigate? Why do they fly in a V-shape? Find out in this week's video!