If you thought birds were amazing before, wait until you hear what they put their bodies through in order to migrate. Although it’s probably no surprise that migration requires a lot of energy, until recently, ornithologists thought migrating birds got all that energy from eating high-energy foods, like berries, along the way. It turns out that the whole process is much more complex. For one thing, before they begin migrating, birds start eating extra food and gain weight. To enable them to take in more food, their digestive system actually changes–its cells grow larger and produce new cells and their gut expands dramatically. But here’s the really fascinating thing. As you might expect, digestion in birds consumes a lot of energy. In fact, the gut is the fastest growing tissue in the bird’s body, since it’s continually digesting and renewing itself.