How Animal Migration Works

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].

Animals, of course, don't have calendars on their walls. What tells an animal when to migrate?

Some species rely on photoperiod -- the amount of daylight in a given day. As the days get shorter, their instincts tell them winter is getting close, so they'd better head south (we'll talk about how they know which way is south later). Experiments have shown that animals exposed to constant artificial photoperiods will act is if they were experiencing those photoperiods in nature [source: Purves].

What if the animal can't see the sun, such as one that hibernates in a cave? Some animals might react to temperature. They could also be responding to internal cues, such as the amount of fat reserves stored in their bodies. Some migration patterns play a careful balancing act -- when fat reserves drop because of dwindling food supplies, it's time to head for a more bountiful wintering ground. But animals need a certain amount of fat for energy to make the journey, so they can't wait too long. Evolution has tweaked these processes so that, barring outside interference, their instincts work perfectly.

In the absence of all external stimuli, many animals still know when to migrate and when to head home. Circadian rhythms and circannual rhythms are internal calendars built into an animal's nervous system [source: Purves]. Although we don't fully understand these rhythms, they are tied to patterns of brain activity that shift with time of day, photoperiod and seasons. Humans have them too, though we don't use them to migrate.

Migratory instincts developed in different species for different reasons, but for the most part they are responses to population pressure. Most migration follows the "leave somewhere cold for somewhere warm, then come back in the summer" pattern. So why would a species live somewhere that got too cold for them in the first place?

The first camp suggests that animals first lived where it was warm all year long, and they didn't need to migrate. As the population grew, resources became scarce. During warm months, northern latitudes were relatively hospitable, so some members of the species expanded their range and began living in these areas. When winter came, food grew scarce and it got too cold for them, so they temporarily relocated to warmed latitudes [source: Drickamer].

The second camp says that climate change is responsible. Species that lived in the north were able to live there all year during periods when the climate was warm enough. As tens of thousands of years passed, however, the climate gradually changed, and eventually the winters grew too cold, forcing the species to head south each year.

The truth about migration is probably a mix of the two, and it probably differs by species. However, the first theory is most likely -- population pressure is the driving force behind most migrations, and, in fact, most evolution. Climate change may have lent a hand in forming or shaping migratory patterns, but it was not the primary force.

Finding the way to wintering sites thousands of miles away is easy for animals -- they just put the coordinates into their GPS systems and follow the turn-by-turn directions. No problem.

Actually, the methods animals use to navigate their migration routes are even more amazing than an animal that could program a GPS device. Some of their navigation methods are so weird we don't really understand them.

The sun - This seems pretty simple. You can judge roughly what direction you're heading in by where the sun is. But factor in the time of day, time of year and cloud cover, and you're left with a pretty tricky navigation system. Yet starlings and ants navigate this way. Some birds can even travel at night using the sun -- theories suggest they take a "reading" from where the sun sets and use that to set their course. Others think that the polarization of light coming from the sun plays a role [source: Purves].

Landmarks - This is another pretty basic navigation system. Fly toward those mountains, head to the left a little when you see the ocean, and make a nest in the first nice-looking tree you can find. Whales traveling in the Pacific Ocean near the North American west coast use this method -- their landmark is hard to miss, because it's the entire continent of North America. They keep it on their left on the way south, and to their right when they head north.

Moon and stars - Planetarium experiments have proved that many birds rely on stellar cues to figure out which way to migrate. We can even tell which star they are orienting from (Betelgeuse, in the case of indigos - [source: Purves]).

Scent - Once an animal is in the general area, scent can pinpoint specific locations. Scent won't get an animal from Saskatchewan to Mexico, but it probably helps salmon find their exact spawning ground, for instance. The scent of rain might shape wildebeest migrations.

Weather - Wind conditions are often used as supplementary navigation aid by birds. When deprived of other cues, such as the sun or stars, birds chose to fly downwind in an experiment [source: Purves]. When the birds could see the sun and stars, they flew in the right direction regardless of wind direction.

Magnetic field - The earth has a magnetic field that's usually undetectable to humans who aren't holding a compass. Some animal species do have the ability to detect the magnetic field, however, and they use it to make their migrations. Bats and sea turtles use magnetic information to find their way [source: PhysOrg]. Some species of bacteria even rely on the magnetic field to orient themselves [source: SAO/NASA].

We're not 100 percent sure how animals detect the magnetic field, but small particles of a magnetic mineral called magnetite have been found in the brains of some species. Those particles may be reacting to the magnetic field and activating nerves in such a way as to send orientation information to the animal's brain.

If you'd like to learn more about animal migration and other topics like it, you might be interested in the links on the next page.­