Animal Migration

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.

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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 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 Animal 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 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.

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