I.

Introduction

Animal Migration, seasonal or periodic movement of animals in response to changes in climate or food availability, or to ensure reproduction. Migration most commonly involves movement from one area to another and then back again. This round-trip, or return migration, may be of a seasonal nature, as in the spring and autumn migrations of many birds. Or it may require a lifetime to complete, as in various species of Pacific salmon that are born in freshwater streams, travel to ocean waters, and then return to the stream where they were born to breed before dying.

Migration occurs in a wide range of animals, from microorganisms in freshwater lakes, which shift seasonally from deep to shallow water as a result of temperature changes; to whales, which move in autumn from subpolar to subtropical seas to have their young and then return in late spring to the colder, food-rich waters. Humans migrate as well: The Kung bushmen of the Kalahari Desert, for example, follow migrating game animals that they rely upon for food. They also leave drought-ridden areas to find other water sources.

In addition to round-trip migration, some migrations are nomadic in nature. Nomadic migrations involve irregular movement patterns that are dependent on temporary local conditions. For example, many of the large grazing animals that live in herds on the plains of eastern Africa move in response to varying local conditions of food and climate. In these migrations, the animals follow no regular route and do not return to any one place. Another type of migration—removal migration, or one-way migration to new sites—is exhibited by migratory locusts of Africa and Asia. These locusts are well known for their enormous mass movements when their populations peak and food becomes scarce. They move to new areas, almost blackening the sky as they pass overhead. Rarely do they return to their place of origin.

Irruption is a specific migratory cycle occurring in extreme climates. The best-known example of irruption is seen in lemmings of the arctic tundra. These small animals reach a peak in their population every three or four years, at which point they migrate overland in great numbers. Most of them die in the process; only a few survive to start the migratory cycle again. Another migration pattern is remigration, in which the round trip is divided between generations: the first generation of animals migrates to an area and reproduces, and the return trip is made by their offspring.

II.

Seeking Food and Water

Migration based on the availability of food is often dictated by seasonal climate change. When winter suddenly halts the supply of insects, for example, birds that eat insects must head for warmer climates where food is still bountiful. Similarly, as the cold settles in, small rodents and birds that are prey for predatory birds become scarce. This scarcity prompts the North American red-tailed hawk, for instance, to fly to Mexico or the Gulf Coast to find a more abundant food source. In winter, animals that depend on fish or aquatic plants from waters in the north find their feeding grounds sealed by ice. These conditions force the animals to travel south in order to survive.

Plant-eating mammals, such as buffalo and antelope, typically graze in herds, which can quickly deplete the grass in an area. In the summer, cropped grass regrows quickly. While waiting for this regrowth, grazing animals may wander a short distance to find new grass, circling back to the original area when grass is abundant again. But in winter, grass does not regrow, which forces these herds to travel longer distances to find fresh food supplies. When spring brings new growth, the herds move back to the areas where they found food the previous season.

When climatic changes cause drought to occur, water holes draw predators and prey alike, making these areas both overcrowded and dangerous. In Africa, wildebeests, zebra, and other prey species therefore migrate to areas where water is more plentiful. Although these animals burn valuable calories to make the journey, the tradeoff is worthwhile because they reduce their dependence on risky watering places.

III.

Reproductive Migration

Another reason animals migrate is to bear their young in places relatively safe from predators and rich in resources. Although reproduction may be the primary factor in these cases, the other elements of food availability and seasonal climate change are often involved as well. Some right whales, for example, leave their Antarctic feeding grounds where their primary food resource of krill (tiny shrimplike crustaceans) is plentiful. They travel to the relatively barren shores of Patagonia, where they bear their young. Although krill are in Antarctic waters year-round, ice covers the surface of the ocean in winter, making it impossible for whales to surface for air.

The green turtle is another reproductive migrant. When the time for laying eggs draws near, female green turtles swim from their feeding grounds off the coast of Brazil. They swim to tiny Ascension Island over 2,000 km (1,242 mi) away, which they may not have visited since they were hatched. After their long swim, they haul themselves onto the sandy beaches, scrape out shallow nests, and deposit their eggs. Once this is done, they swim back to Brazil.

Freshwater eels spend most of their lives in the rivers of North America and Great Britain. However, for reproductive purposes, they trace ancient migratory patterns, swimming from each side of the Atlantic to the weedy Sargasso Sea between Bermuda and Puerto Rico. After breeding in the Sargasso, the eels return to the continental rivers. The young eels, or elvers, take a year or two to reach American shores, and they are often three years old before they reach European rivers. This movement between saltwater and freshwater involves a special migratory adaptation—a physiological shift in kidney function. Without this adaptation, eels could not make this dramatic change in environment without bodily harm.

IV.

Prerequisites for Migration

In order to migrate successfully, animals must be capable of sustained movement for long periods of time. This movement requires considerable energy output, and different species of animals have evolved a variety of mechanisms to ensure sufficient body fuel for the trip. How an animal migrates is significant in determining how much energy is needed as well as how much must be stored in the body. Flying, for example, is more physically intensive than walking or swimming, so migratory birds must build up large energy stores before they set off. Just before spring and fall migrations, certain birds increase their body fat—up to nearly 40 percent of body weight in some songbirds. Before migrating, the 10-cm- (4-in-) long ruby-throated hummingbird gains about 2 g (0.07 oz) of fat. This extra fat provides sufficient energy for this tiny migrant to fly 800 km (500 mi) from North America, across the Gulf of Mexico, to its winter home in Mexico. Some birds supplement this stored energy source with food along the way. Others make long, nonstop flights—the golden plover may travel 3200 km (2000 mi) over water without landing.

In contrast, land mammals are able to graze on plants along the way as they travel, so they have no need for sizeable fat stores. In fact, it is important that these animals travel light so that they can remain agile to escape predators. Land mammals, such as African wildebeest, may walk more than 1600 km (1000 mi) when migrating.

V.

How Animals Migrate

The mystery of animal migration remains one of the most compelling in science. How do animals know when it is time to migrate? How do they find their way back to the same place year after year? How do they travel unerringly to a certain beach or stream that they have not seen since birth? Some of the animals' secrets are beginning to come to light with the help of scientific observation and experimentation.

Most migratory animals are subject to internal signals that prepare them for migration. Many migrants develop large appetites at the beginning of the migratory season, causing them to increase their food intake and accumulate fat stores. This overwhelming urge to eat is triggered by hormones secreted by the pituitary gland, located in the lower part of the brain. This gland also controls the development of the sex glands, which produce sex hormones and reproductive cells. In this way the pituitary gland guides the animal toward both migration and reproduction in subtly interconnected rhythms. Once these internal signals have physically prepared the animal for the journey, the animal senses certain external cues—the temperature drops or food becomes scarce—and the migration begins. Significantly, these inner hormonal changes do not occur in animals that do not migrate.

As they begin their journey, migrating animals use a variety of specialized abilities and senses to reach their destinations. At the simplest level, animals rely on external forces, such as wind or water currents, to propel them to their migratory goal. Sparrows in eastern North America, for instance, catch prevailing winds that carry them to South America. Similarly, baby eels emerging from the Sargasso Sea ride on water currents to reach the mouths of rivers in North America and the United Kingdom.

Other animals use more complex instinctive mechanisms to navigate. Some animals rely on familiar landmarks, such as coastlines, mountain ranges, and river valleys, to follow a specific route. Mature salmon depend on olfactory cues, or odors, in their migrations. Salmon memorize the odor of their home stream on the day they begin their migration to the sea as juveniles. Years later they navigate back from the ocean to the mouth of their home river and track its distinctive odor upstream.

Among other specialized senses are an internal biological clock, found in virtually all animals, that enables them to track the passage of days (or even months). Migratory animals combine their remarkably accurate sense of time with cues from the sun to determine their exact location and to travel in the right direction. For example, when an animal in the northern hemisphere senses that it is midday, it recognizes that the sun is due south and uses this information for orientation. Some animals take this a step further, using the position of the sun and special patterns of reflected sunlight to determine orientation and direction. These light patterns make it possible for animals that rarely look at the sun directly—fish, for example—to use the sun to get their bearings.

Some animals migrate at night, when predators are less of a threat. After dark, the stars, rather than the sun, provide orientation for these animals. Birds learn the pattern of stars in the sky and are able to discern true north even when only part of the sky is visible. These methods of orientation and navigation are sometimes called sun compass and star compass, respectively, and they closely resemble the techniques of celestial navigation used by sailors in earlier times.

Some birds, notably pigeons and sparrows, display the ability to find their destination even if they are taken off course and far from their navigational cues. Recently, scientists have discovered tiny crystals of magnetite—a magnetic substance—in the brains of some animal species. The scientists believe the magnetite enables animals to use the earth’s magnetic fields as a guide. This magnetic compass may explain the strong directional sense of aquatic migrants, such as whales, sharks, trout, and sea turtles, who rarely use the sun or stars for guidance.

There is also evidence that certain chemical signals in animals help to trigger or guide migration. For example, many one-way migrations, such as the overland run of lemmings or the swarming of bees, are initiated by pheromones—chemical signals released by an animal that affect the behavior of other animals of the same species.

For many animals, migration routes are inborn. Monarch butterflies, for example, summer in temperate zones of the United States and southern Canada, and then winter in Mexico. As they head south, the Monarchs fly without guidance or previous experience, relying entirely on innate directional cues. For other species, however, learning is crucial. Young geese learn migration routes in groups, benefiting from the navigational experience of older birds. In addition to learning the route, the geese also learn migratory flying strategies, such as flying in “V” formation. This formation enables the uplift of air from the leader bird’s wings to give the birds following behind an aerodynamic boost.

For animals reared in captivity and then released into the wild, learning their species’ migration patterns can be difficult. Large water birds like geese and cranes must be led to wintering grounds for the first time by their parents. They do not use the sun, star, or magnetic cues.

In an effort to reestablish populations of whooping cranes and sandhill cranes in protected areas, scientists have reared young birds with the intention of teaching them where to migrate in winter months. Researchers have created several ingenious methods of providing these fledglings with “parents” that lead them on migrations. These methods include flying ultralight aircraft to lead the birds south to their winter homes; building larger-than-life, radio-controlled robots that look like cranes for them to follow; and training the young birds to follow trucks or other land-bound vehicles.

VI.

Hazards of Migration

The hazards an animal faces when migrating fall into two categories—natural and human-made. Natural hazards include climatic changes, drought, food scarcity, predators, and the individual physical demands of migration on the animal. In some cases, the animal’s migratory behavior poses a considerable hazard as well. In southern Africa, for example, springbok migrate in herds so dense that death from trampling, starvation, or drowning is not uncommon. Other animals caught in the springboks’ migration path suffer as well, often being swept along or trampled by the tide of rushing bodies.

Throughout history, humans have posed particular dangers to migrating animals. The predictability of animal migration routes makes many migrants vulnerable to human intervention. For example, the caribou of arctic regions are hunted by Inuit who intercept herds along seasonal migration routes. Sport hunters acquaint themselves with migration routes as well. In the fall, for instance, goose and duck hunters go to specific feeding grounds along known migratory routes of these North American birds. The hunters wait at these feeding grounds and shoot the birds as they fly overhead on their way south for the winter. Elk who customarily migrate to lower elevations of mountain ranges in the winter in search of food become quarry for hunters who anticipate their movements.

Human-made structures also take a toll on migrants. Skyscrapers and radio towers have caused the deaths of hundreds of thousands of migrating birds. Dam construction in certain areas has made it impossible for fish to swim upstream to spawning areas. In recent years, various measures have been developed to protect migrants from such interference and needless death. For example, fish ladders—concrete steps with water sluicing downwards—built alongside dams can provide a safe parallel passage for spawning fish to reach upstream waters.

VII.

How Migration is Studied

Determining how animals migrate is challenging for researchers because migrating animals are on the move, not sitting quietly in laboratory cages. Scientists can track large herds easily enough, following the route in land-bound vehicles. Researchers also follow migratory behavior on an individual basis by tagging individual animals with identifying markers. For instance, scientists fit a small plastic band around a bird’s leg to confirm if that particular bird returns to its customary wintering ground. Both land and marine animals, such as deer or dolphins, can be outfitted with collars containing electronic transmitters. These transmitters send radio signals to researchers, enabling them to determine an animal’s exact location at any time, as well as its overall migration route.

Scientists have used radar to prove that migrating birds “fly true”—in a straight line in the correct direction—at night, even through thick clouds. Some scientists have followed individual birds in airplanes. Even satellites have been enlisted to spot animals, such as elephants that have been equipped with specialized transmitters. But finding out how animals orient and navigate requires many different methods of observation.

Homing pigeons do not migrate in the strict sense of the term, but scientists have studied them extensively because of their impressive navigational powers. These pigeons can be taken hundreds or thousands of miles from home, and when released, they are able to return confidently to their home loft. Pigeons fitted with frosted contact lenses that cloud their vision can nevertheless fly to within half a mile of their loft. Scientists theorize that they locate their lofts by following some combination of magnetic and olfactory cues.

Wild birds kept in cages during migration season indicate the direction they want to fly by hopping repeatedly on mechanized perches. When these cages are placed in planetariums, the birds rotate their direction with the movement of the artificial stars overhead. This response confirms that birds depend on stars for orientation. If the artificial stars are removed, the birds orient themselves to magnetic cues.

Much remains to be discovered about the mechanisms animals use when they migrate. How do young birds, such as the Arctic tern, for instance, adjust to the new constellations they see when they migrate across the equator into the southern hemisphere for the first time? How do animals use the minute amounts of magnetite in their brains to sense the earth's magnetic fields and then decode this information? As a phenomenon that uses abilities we are just starting to understand and senses that we lack, animal migration remains one of nature's best-kept secrets.