It’s tempting to compare bird migration to marathon running. In both, participants prepare intensely and undergo an extreme test of endurance. But the similarities stop there. Though marathon runners push the human body to its limits—during the 26.2-mile race, core temperatures spike to 102 degrees Fahrenheit and the heart pumps three to four times more blood than usual—birds radically change their bodies and their metabolism for the main event. In just weeks or months, they undergo physical transformation unmatched by human gains from years of training. To fly vast distances between breeding and wintering grounds, birds can shrink their internal organs, rapidly gain and burn through fat stores, barely sleep, and more. These are incredible abilities, but they come with tradeoffs. The energy required to fly hundreds or thousands of miles in a short span leaves birds with little room for error during migration, and vulnerable to natural and human-caused threats. In North...