In the past two decades, our understanding of the navigational and physiological feats that enable birds to cross immense oceans, fly above the highest mountains, or remain in unbroken flight for months at a stretch has exploded. What we've learned of these key migrations--how billions of birds circumnavigate the globe, flying tens of thousands of miles between hemispheres on an annual basis--is nothing short of extraordinary.Bird migration entails almost unfathomable endurance, like a sparrow-sized sandpiper that will fly nonstop from Canada to Venezuela--the equivalent of running 126 consecutive marathons without food, water, or rest--avoiding dehydration by "drinking" moisture from its own muscles and organs, while orienting itself using the earth's magnetic field through a form of quantum entanglement that made Einstein queasy. Crossing the Pacific Ocean in nine days of nonstop flight, as some birds do, leaves little time for sleep, but migrants can put half their brains to sleep f
