In the mountains of central Mexico, monarch butterflies gather each winter in one of the most dazzling displays of mass movement in the animal world. Many millions of them migrate there from across North America to escape the cold before traveling north again in the spring.
In the neotropics, nocturnal bats fill many of the niches occupied by birds by day. But where birds use their superb sense of sight, bats exploit their specialized sense of hearing to find prey. They produce high-frequency clicking sounds and listen with finely tuned ears for the echoes—then strike.