However, when birds migrate, they can’t afford to spend all that energy on digestion. And so their digestive system partially shuts down and stops renewing itself, so all that energy can be diverted into fueling flight. In other words, migrating birds end up using their own body tissues as fuel. Then, when they arrive at their destination, the birds eat foods that are high in protein so that their digestive system can rebuild itself and return to normal, which usually takes a few days. This need for protein-rich food is something ornithologists didn’t always know about. Given that less than half of the birds that leave don’t make it back, the more scientists know about the stresses of migration, the better.