At the heart of all these different forms of migration is one instinct: survival. Most migrations allow a species to prosper, by leaving an area when there isn't enough food there to support the population. They also prevent the long-term depletion of food sources in one area. These periodic movements mean each individual has a better chance of finding enough to eat.
While food-driven migrations can be very regular, there are many variables that can affect the availability of food, including weather and the population levels of other species sharing the same territory. For this reason, some species use irregular migration patterns that constantly shift and adapt to new conditions. Wildebeests travel across the African plains in search of water to drink. When their regular water sources dry up, they head into the brush in search of grass and more water. Their dry-season migrations can be altered by the sound of thunder and sight of rainclouds [source: Bolen].
Migration patterns also benefit mating and breeding, allowing young to be born in regions with richer food sources or far away from dangerous predators. Chinook salmon and other related species are born in rivers in the U.S. northwest, then head out to sea as adults. Later in life, they swim back upriver to mate and lay eggs in the exact same place where they were born. Young salmon would be too vulnerable to predators in the open ocean, and returning to their original spawning ground ensures that the eggs are laid in a successful spawning point (after all, it was good enough for the parents to spawn and survive to adulthood). Dams along their spawning rivers cause serious problems for salmon, and their populations have dropped drastically as a result [source: Audubon Magazine].
Some migrations are driven by both food and reproduction. Baleen whales, a category that includes gray whales, blue whales, right whales, Minke whales and humpback whales, travel north in the summer (or south if they live in the southern hemisphere). In cold polar waters they find vast quantities of their favorite food -- krill, tiny shrimplike creatures. But young whales don't have enough blubber to insulate themselves from the cold, so the whales return to tropical waters each winter to give birth [source: Bolen]. Migration routes and distances vary by species, but many travel thousands of miles. Gray whales migrate more than 6,000 miles one way [source: SeaWorld/Busch Gardens].