A look at some of the planet's great gatherings, creatures that come together in inconceivable numbers - sometimes in millions, billions, and even trillions. Included are bats and bees, locust and ants, monarch butterflies in Mexico, 17-year cicada hatches, grunion in the Sea of Cortez and carp in the Mississippi River, sardine runs off the coast of South Africa, super flocks of parakeets in the Australian Outback, mayflies on the 4th of July, and even penguins and wildebeest. Some gather to bre...