How Animal Migration Works
There are few sights as majestic as the mass migrations of animals. To humans, thousands of monarch butterflies roosting in trees or scores of wildebeests surging across the African plains are beautiful and impressive to witness. To the animals themselves, these migrations are crucial to the survival of their species. In this article, we'll learn about the biggest, longest and most amazing animal migrations in the world, and we'll figure out why and how animals make such astonishing journeys.
Arctic Animal Image Gallery
Johnny Johnson/Getty Images
Porcupine caribou herd migrates in winter at the ArcticNational Wildlife Refuge in Brooks Range, Alaska.See more arctic animal pictures.See more arctic animal pictures.
Migration is the large-scale movement of an animal species from one place to another. Migrations are usually tied to seasonal changes in weather and feeding patterns, or mating and breeding patterns. Some migrations don't follow these patterns. Irruptive migrations don't seem to follow any pattern at all, and nomadic species move from place to place whenever they've exhausted the food supply in one area [source: National Park Service].
When every member of a species migrates, that is known as a complete migration. If some members of a species stay in one place all year long, but others migrate, that's a partial migration. This usually occurs when the species' range is large enough that some individuals live someplace that is always relatively warm, while others live in a temperate region that gets too cold in the winter. Barn owls are one example of an animal that partially migrates [source: Bolen].
Animals that live in mountainous areas may use altitudinal migration. Instead of traveling long distances, they move to lower altitudes when winter snow hits the upper mountains. Some spotted owls do this, while others of the same species who don't live in mountains have different migration patterns [source: The Condor].
A drastic change in the environment or climate can result in removal migration. If a species' habitat becomes permanently inhospitable to them, such as when human development drains swampland or clear-cuts a forest, the species will attempt to move to a different area and not return to their original home.
Caribou, a large species of deer-like animals native to northern climates, hold the world record for the longest overland migration. Each year, 3 million caribou make seasonal journeys across the Arctic tundra. They literally seek greener pastures, always traveling to find fresh grazing grounds. The distance traveled varies by herd, with larger herds traveling farthest. Some cover more than 2,000 miles each year [source: Arctic National Wildlife Refuge, BBC].
Although no carnivorous mammals have regular migrations, some wolf packs may travel for some distance with a caribou herd if food becomes scarce in the pack's home territory [source: Bolen].
Why Migrate?
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].
Humpback whale and calf in the South Pacific, Tonga, Vava'u
Doug Allan and Sue Flood/Getty Images
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].
Whooping CraneThe whooping crane is the tallest bird in North America, but habitat destruction virtually wiped out the entire species. At one point, fewer than 20 birds remained in the wild [source: Cornell Lab of Ornithology]. The entire eastern population of whooping cranes vanished. Western cranes have recovered somewhat, but biologists wanted to reintroduce whooping cranes to the eastern United States. This presents a bigger problem than simply moving a few crane families. Cranes learn how to fly to their winter feeding grounds when they're young by following their parents. With no cranes living in the east any more, there were no older cranes who knew the way. So scientists developed a clever method of teaching whooping cranes how to migrate.
Crane chicks born from captive populations are reared by humans wearing crane "costumes," and they're acclimated to the sound of an ultralight aircraft. When the time comes to migrate, the birds are led on a 1,200-mile journey by a human pilot in an aircraft. They travel from Wisconsin to Florida. It's hoped that once the cranes learn the migratory route, they will successfully teach it to their chicks and reestablish an eastern flock that doesn't need human migration guides [source: U.S. Fish and Wildlife Service].
Migratory Cues
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.
Sooty ShearwatersThe sooty shearwater is a bird native to New Zealand, where it breeds during the southern hemisphere's summer. When the weather gets cold, sooty shearwaters head north, all the way to the North Pacific. They spend the warm northern summer living on the coasts of California, Japan and Alaska. This migratory pattern is notable because it's the longest ever confirmed by an electronic tag. A sooty shearwater's mileage for a single year's migration (up north and back) can exceed 40,000 miles. It's thought that they ride massive wind currents to ease the journey [source: National Geographic].
The arctic tern may actually have a longer migration route than the shearwater, moving from the Arctic to the Antarctic and back. It racks up about 500,000 frequent flier miles in a lifetime [source: British Trust for Ornithology].
The Evolution of Migration
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.
Monarch ButterfliesMost insects hibernate during the winter months, but a few species migrate -- none so spectaculary as the monarch butterfly. As autumn nears, butterflies begin to cluster in their summer homes throughout North America. They begin heading south toward their overwinter sites in Mexico, a journey of more than 1,000 miles. There they roost by the thousands in trees. In the spring, they return home.
The annual monarch migration is not just beautiful, it's also fascinating and mysterious. The migration itself lasts longer than a typical monarch butterfly's lifespan. The monarchs that make the trip slow down their metabolism so that they live long enough. That means that each migrating monarch will never make the trip again. In fact, two or three generation of monarchs will pass before another group migrates. Yet the offspring of each migrating monarch follows the same flight path and sometimes roosts in the same tree as its great-great-grandparents. No one is sure how they find the way, though they may be using the sun, landmarks, or scent [source: Washington NatureMapping Program].
Animal Navigation
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]).
Wildebeest crossing creek bed in the Masai Mara, Kenya, Africa.
Grant Faint/Getty Images
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.
Sea TurtlesBaby loggerhead sea turtles are able to find their way along an 8,000-mile migration route the first time they ever see it. Scientists took some turtles off course, but they were able to find their way back with little difficulty. Believing that some magnetic orienteering was going on, the next experiment subjected the turtles to a variety of magnetic fields that differed from the earth's natural field. These turtles went off course. Exposure to a magnet that mimicked the earth's field set them right again -- proof that the turtles can detect the earth's magnetic field and use it to navigate [source: ScienceDaily].