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Migration
Kenneth P. Able 

Many animals spend their entire lives within a small area. Others continually roam over vast ranges. Still others travel immense distances between areas that they inhabit for part of each year. An area in which an animal lives for an extended period of time is called its home range. The home range typically provides food, shelter, nesting or breeding sites, and suitable conditions for raising young. If any of these resources are unavailable for part of the year, an animal may have to move to another area where living conditions are more suitable. This movement is called migration.

Homing
Animals that migrate often return to the same sites year after year. When animals return to a location where they have lived before, we say that they are showing site fidelity. The process of returning to that place is called homing. Migrating animals use sophisticated navigation methods to find their way. But even animals that stay within their home range use homing methods. These animals typically have an area within their home range that provides safety and shelter. They use various navigation strategies to return to their safe spots when threatened or when returning from searching for food.

Some animals use visual clues to home. A chipmunk whose burrow is in the woods beside your house might routinely visit the bird feeder in your yard. Over the course of its life, the animal will become so familiar with the landmarks and other features of your yard that it will be able to dash immediately to its burrow if a hawk, cat, or dog threatens.

Ants use a different strategy. Ants are social insects that live in large colonies. Worker ants fan out from the colony in search of food, and when they find a rich supply, they deposit a trail of chemicals called pheromones along the ground as they return to the colony. Other workers will then follow the pheromone trail out to the food and back.

Seabirds such as albatrosses spend most of their lives over the open ocean, but they return to land to nest. Nesting typically takes place on islands and the parents must fly long distances out to sea to gather food to bring back to the growing chicks. These round trips may exceed 9,300 miles (15,000 kilometers) and take more than a month to complete. Scientists are still working to discover the ways that albatrosses navigate on these food-gathering journeys.

The most dramatic cases of homing involve animals that travel very long distances -- the migratory species. Migration can mean different things for different types of animals. Birds, whales, and monarch butterflies make seasonal, round-trip migrations. Some desert-dwelling animals exhibit nomadism -- they migrate in search of water and other resources but do not return to the same place, so exhibit no homing. This article focuses on migration that involves homing.

Why Animals Migrate
Animals migrate to cope with a world that changes. Often the change is seasonal. For example, in some areas the summers provide an environment in which animals can easily survive and reproduce, but the winters are harsh and few resources are available. Thus, as winter approaches, certain animals migrate to another place where the necessities of life can be found. (Other animals survive the winter by hibernating.) Animals also migrate if the areas that provide good sources of food are not good places for breeding and giving birth. In this case, animals move to more suitable areas for the breeding season and then return with their young to areas where food is abundant.

What Animals Migrate?
Migration is an option for animals that can fly, walk, or swim long distances. Only three living groups of animals -- insects, birds, and bats -- possess the ability of true flight. Long-distance migrants can be found in each of these animal groups as well as in species of fish, whales, and a number of other sea and land creatures.

Birds
Birds make up the largest number of migratory species and they travel the greatest distances. Birds are very diverse in their migratory habits. Some species, such as the Arctic tern, span the globe in their annual travels. Arctic terns nest on beaches and tundra across the Arctic in both the Old and New Worlds. When winter arrives in the Northern Hemisphere, the terns migrate southward to Antarctic waters where it is summer. As a result, they spend most of their lives in the long days of summer and thus see more daylight in a year than any other organism on Earth. During the course of a round trip, these birds travel 18,600 miles (30,000 kilometers) or more. Since individual terns may live 25 years, one bird may cover close to 500,000 miles (800,000 kilometers) during its lifetime.

Although their trip is very long, Arctic terns can rest on the water and obtain their food from the ocean's surface. Other birds, such as the blackpoll warbler, make nonstop flights on their migrations. During the breeding season, the blackpoll warblers' range spans North America from eastern Canada to Alaska. In autumn, the birds gather along the coast from Nova Scotia to New Jersey. Here the birds prepare for their flight by storing up fat that will serve as fuel. When weather conditions are good, the blackpoll warblers take off on a flight over the western Atlantic Ocean to South America. Unable to land on water, the birds must fly continuously for up to four days to complete the journey.

Many bird migrations are less dramatic, but no less effective. Large numbers of waterfowl move from nesting areas in the north central United States and southern Canada to spend the winter along the Gulf of Mexico coast. Other birds simply move from high elevations in the mountains down to the milder winters of the valleys.

Butterflies
A few species of butterflies are long-distance migrants. The most famous is the monarch butterfly of North America. There are two major populations of monarchs -- one that spends the winter at coastal sites in California, and the other that overwinters in a few small valleys in central Mexico. In the spring, the Mexican monarchs migrate northward into the United States, laying eggs on milkweed plants as they move. The adults, now as much as ten months old (very old for a butterfly), die off before they reach the northern states. Their places are taken by young butterflies that hatch as caterpillars from the eggs laid during the first part of the northward migration. These new adults also migrate northward, and by the end of the summer, successive generations of butterflies have reached Canada. As the days start to shorten, the butterflies begin to migrate southward. Although these individual butterflies have never been to Mexico -- all those that spent the previous winter in Mexico have died -- they somehow find their way to the tiny valleys that are the wintering areas for their population.

Migrations in the Oceans
The climate in the oceans is more stable from season to season than that on land, but there are large differences in conditions from one part of the ocean to another. It is these differences that drive the migration patterns of many animals that live in the sea.

Several of the great whale species migrate. Migratory gray whales can often be observed from shore along the coast of California. Birthing and rearing young calves is best done in warm water; however, the ocean regions with warm water are not usually the richest places for food. So the whales migrate northward to the cold, food-rich waters near Alaska to gorge themselves during the summer. Then they migrate southward to the warm lagoons on the western coast of Mexico, where they fast for months, give birth, and mate again.

Other marine mammals, such as northern fur seals and elephant seals, also migrate considerable distances. They search for food at sea, but must return to land in order to give birth to their young. They often swim several thousand miles between their ocean feeding areas and the breeding islands and beaches to which they return year after year.

Sea turtles also migrate. They nest on land, but the rest of their lives are spent at sea and they travel great distances within the major oceans of the world. Each year, nest-building and egg-laying occur on beaches that have been used by turtles for as long as historical records exist. Most famous is Ascension Island, which is only about 6 miles (10 kilometers) across and located in the center of the South Atlantic Ocean. The turtles migrate from feeding grounds along the coast of Brazil to Ascension Island, a distance of about 1,370 miles (2,200 kilometers). They arrive each December to begin the seven-month nesting period. Satellite tracking has shown that the turtles follow nearly straight paths between Brazil and the island.

In the continental shelf waters around the Bahaman Islands, spiny lobsters engage in mass migrations. Their movements are seasonal and seem to be driven by the animals' need to escape cold water temperatures. When the first severe autumn storm occurs, the lobsters form long lines and migrate by walking across the seafloor day and night until they have reached deeper and warmer water where they spend the winter.

Salmon
Salmon spend the first part of their lives in freshwater streams and rivers and then migrate to the ocean. Years later they return to the same streams in which they hatched to lay their own eggs. Pacific salmon of several species lay their eggs in the gravel beds of streams that may be hundreds of miles from the ocean. The eggs hatch there, and the young fish develop in the stream. As they grow, salmon migrate downstream and eventually find themselves in the large rivers that empty into the Pacific Ocean. For three or more years they live in the sea, moving long distances in search of the smaller fish they feed upon. When they reach reproductive age, the salmon return to the mouth of the river system in which they were born. It is not known how the salmon navigate in the open ocean or how they locate the correct river system, but once they are in the river, the salmon are guided by smell. At each fork in the river, the fish select the correct stream. They seek out the odor of the river in which they were born, which was learned when they lived there shortly after birth. Pacific salmon breed only once in their lives, and they die shortly after the migration. Adult Atlantic salmon return to the ocean after breeding and may repeat the round-trip migration several times during their lives.

Migrations on Land
Most land mammals do not migrate. But some large hoofed mammals do. Caribou, for example, range from Alaska across Canada. Some herds remain in one place all year, but others migrate in large groups of 20,000 to 130,000. They spend the summers on the birthing grounds of the arctic tundra eating leaves and mushrooms. When winter comes, they move south to the sparse forest regions, where they eat mostly lichens. Caribou travel over a distance of about 100 to 600 miles (160 to 960 kilometers). The path that the caribou follow is not always the same because they use different areas of their winter range each year. Migration routes may also be altered by human-made structures such as highways.

In Africa, wildebeests engage in one of the largest land migrations on Earth. In the rainy season the wildebeests feed on the short grass of the Serengeti in Tanzania. When the rainy season ends, usually in late May, the water pools and grass dry up, and the wildebeests begin to move northwest toward Lake Victoria and Kenya's Masai Mara game reserve. The wildebeests usually reach the game reserve between August and October. In November they move south again as the rainy season begins. Almost 2 million wildebeests migrate together, along with zebras and other hoofed mammals that follow a similar route.

The Timing of Migration
All living organisms possess internal biological clocks. Best known are the daily, or circadian, clocks, but many animals also have circannual clocks that have a period of about a year. The internal clock triggers many of the events that occur during a year of the animal's life -- events such as migration or hibernation, molting, reproduction, and so forth.

In the months prior to migration, animals begin to prepare for their journeys by eating more food and storing up fat. This is called coming into migratory condition. Animals will come into migratory condition even without a signal from the external environment. This has been demonstrated in the laboratory with animals maintained in unchanging environmental conditions.

In the real world, there are many signs of the changing of the seasons. The most important to migrators is the change in the length of daylight (photoperiod) across the seasons. (The days get longer from winter to summer and shorter from summer to winter.) Changes in photoperiod keep an animal's internal circannual clock in phase with events in the outside world. A migratory bird kept in captivity can be stimulated to come into migratory condition in the middle of winter if the day length is gradually increased using artificial lighting.

How Do Animals Navigate?
Migrating animals sometimes travel very long distances. How do they find their way? Animals seem to use several methods to navigate. Returning to a home site might involve simply turning around and following the path of the outward journey back to its beginning. Animals that rely on vision might learn conspicuous features of their surroundings and then retrace their paths by following the sequence of familiar landmarks. This is called piloting.

A more complicated navigation mechanism is path integration, or dead reckoning. As an animal moves away from home, the animal's brain logs information about the directions, turns, and distances traveled and then integrates this information to compute a direct course back to the starting point.

Animals that travel very long distances cannot memorize enough information to be able to navigate on the basis of information obtained during the journey away from home. Instead, these animals have what are known as compass sense and map sense. To understand these internal navigation systems, imagine yourself blindfolded and taken by car to a distant place that is completely unfamiliar to you. There the blindfold is removed and you are given a compass and asked to find your way back home. You will not be able to do so, because although the compass will tell you which way is north, south, east, and west, knowing these compass directions will not be useful unless you know which direction is toward home. In order to be able to navigate back home, you need both a map to tell you the direction in which home lies, and a compass to identify that direction. Using internal compasses and maps, an animal can determine where it is with respect to home, even from considerable distances.

Animal Compasses
Animals possess a number of compasses. Many insects, crustaceans, fish, amphibians, reptiles, birds, and mammals are able to sense the Earth's magnetic field and use it to determine compass directions. Exactly where these magnetic sense organs are located and how they work are not completely known, but it appears that these animals have specialized cells that contain magnetite crystals. These crystals are magnetic and line up with the Earth's magnetic field just as the needle on a compass does. The position of the crystals is somehow signaled to the brain.

Many species of both invertebrate and vertebrate animals also use the sun as a compass. Doing this requires knowing the time of day and understanding the path of the sun as it moves across the sky -- that is, that it rises in the east, passes through the southern sky at midday in the Northern Hemisphere, and sets in the west. Birds must learn how the sun changes position during the day; insects are apparently born with genetically coded information about the sun's path.

Some animals can get directional information from the sun on cloudy days as well. Light from the sun is polarized by the Earth's atmosphere. This means that as sunlight is scattered through the atmosphere, light rays vibrating in certain directions are allowed to pass through while others are absorbed. This results in a specific pattern being formed in the sky. Some animals recognize patterns of polarized light and use them as a compass.

Birds that migrate at night use a star compass. During their first few weeks of life, they learn the star patterns and how they move across the night sky. By the time they embark on their first migration, they know the constellations in much the same way that you might. By observing the way the positions of stars change during the night, the birds can identify the North Star, which does not move, and thereby know the direction of true north.

Animal Maps
In addition to internal compasses, animals have other means of finding their way home. The Earth's magnetic field can provide maplike information in addition to compass directions. For example, the field is weakest near the equator and strongest at the magnetic poles. If an animal can detect very tiny changes in the field, it could use such patterns to figure out its position with respect to home. Doing so requires much greater sensitivity to the magnetic field than is necessary to use the field as a compass. There is good evidence that sea turtles and newts can detect such small changes in the field. Homing pigeons may be able to do so as well.

Homing pigeons and other animals may also rely on olfactory (odor) maps. This does not mean that they can smell home from great distances. Rather, they seem to learn the odors of the region in which they live by smelling different things as winds come from different directions. In this way they learn, for example, that they smell the sea when the wind blows from the west, a pine forest when the wind comes from the east, and so forth. If a homing pigeon is carried away from home by a scientist performing a homing experiment and smells a strong odor of pine where it is released, it will conclude that it is east of home.

There are still many unanswered questions about animal migration. Scientists remain fascinated by the skill with which these animals can return to the exact locations year after year, and continue to investigate their navigation strategies.
   
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