no one knows why they do it. yet each fall, thousands of starlings dance in the twilight above gretna, scotland. the birds gather in magical shape-shifting flocks called murmurations, having migrated in the millions from russia and scandinavia to escape winter’s bite. scientists aren’t sure how they do it, either. even complex algorithmic models haven’t yet explained the starlings’ acrobatics, which rely on the tiny bird’s quicksilver reaction time of under 100 milliseconds to avoid aerial collisions—and predators—in the giant flock.