At any moment somewhere in the world millions of migratory animals are on the move. An extraordinary variety of species embark on long and difficult journeys across land, through rivers and oceans, and in the air. Migration can be complex and difficult to comprehend. How do animals manage to travel so far and with such navigational accuracy? What is the strange force that their ultimate destination seems to exert on them? This phenomena has captivated human beings for thousands of years, ever since Paleolithic hunter-gatherers learned to follow herds of hoofed mammals across the grassy plains of what is now Africa and southern Europe. It is only within the last 150 years, and the past few decades in particular, that zoologists have really begun to reveal the hidden secrets of this fascinating animal behavior. Rapid developments in consumer electronics and mobile communication are today driving a revolution in migration studies. Satellite telemetry, which enables researchers to pinpoint the location of an animal tagged with a radio transmitter, is now so advanced that it is practical to follow the movements of individual creatures, almost anywhere on the Earth’s surface, virtually as soon as they happen. Some examples are the sooty shearwater, a globe-trotting seabird that has been tracked across the entire Pacific Ocean as it traced a huge figure-of-eight pattern or a 39,800 mile round trip from its nesting site in New Zealand, a leather back turtle that was monitored for 21 months on the longest recorded migration (12,774 miles) of any aquatic species, a great white shark tagged off the coast of South Africa that swam 12,400 miles to Antarctica and back again in less than nine months. As data like these are collected, a better understanding of animal migration has resulted.
Migration exists in an abundance of forms. But one thing unites all migratory animals, the fight to survive. While dangerous, it is a means of staying alive. The participants have evolved sophisticated ways of reducing the risks, so that although some undoubtedly will perish, as many complete the trip as possible. This paper will discuss some of the greatest travelers in the animal kingdom and at the end suggest how to experience some of them at first hand. For the less adventurous, migratory wildlife can usually be enjoyed close to home, even in one’s own backyard. Migration takes many forms. It is much more than a simple trip from A to B, and migratory journeys are as varied as the animals that perform them. Some of the most intriguing questions in migration studies are how animals get ready, know when to set off and where to go, and navigate without getting lost.
What is Migration?
Animals make all kinds of different movements, short and long, seasonal and daily, regular and once-in-a-lifetime, highly predictable and seemingly random. It is not always easy to decide which ones are true migrations. The classic idea of migration, and certainly the most widely held, is of flocks of birds flying north and south between separate breeding and nonbreeding ranges in tune with the ebb and flow of the passing seasons. Many species from diverse animal groups do conform to this general migratory pattern, but it represents only one type of migration. There are numerous others, including journeys between east and west, complex circuits of land and ocean, seasonal trips up and down mountains, and neutral movements through the water column of seas and lakes. In addition, members of a particular species may follow a wide variety of migratory routes, and in some migrations only a portion of a species population may be involved. However, for purposes of this paper, migration is categorized as a journey with a clear purpose from one area or region to another, often following a well-defined route to a familiar destination, and often at a specific season or time.
Migration is crucial for survival. It has evolved to enable animals to spend their life in two or more different areas, usually because a lack of food or a period of extreme weather makes it impossible to remain in the same location permanently. Other common reasons to migrate include: to find water, or essential minerals, to hunt for a mate, to give birth, lay eggs, or raise young in a safe place, and to avoid predators or troublesome insect parasites. Animal migrations may be driven by several factors simultaneously.
If an animal remains in its present situation, when conditions become less favorable, it has three alternatives to migration. First, it can adopt behavioral changes, such as in its diet or the shelter it uses. Second, it can undergo morphological (bodily) changes, by growing thicker fur or plumage. Third, it can fall into a deep sleep, called hibernation. In practice, however, these strategies are not a realistic proposition for large numbers of animals, which forces them to migrate instead. Lastly, it is best to regard some routine animal journeys as “almost” migrations rather than migrations in the strict sense of the word. For example during the breeding season many parent animals temporarily leave their young to go on feeding trips. These sorties may last up to a day in the case of seabirds such as boobies and garnets, or about three to five days in seals and walruses. Land carnivores, including wolves and spotted hyenas, are also long-distance commuters while they have offspring to provide for, frequently traveling dozens of miles to bring back fresh meat for their hungry cubs.
The Circle of the Seasons
An important feature of the global climatic system is the dramatic shift in climate according to latitude. Areas near the poles (high latitudes) have long, dark bitterly cold winters followed by short intense summers, whereas equatorial areas (low latitudes) enjoy high temperatures and a steady supply of sunshine all year round. These stark contrasts are caused by the fact that the Earth’s rotational axis, on which it spins once each day, is tilted 23.5 degrees from the vertical with respect to the plane of its orbit around the Sun. The seasonal cycle produces huge differences in the duration and intensity of solar energy received in each hemisphere at any given time and has caused many species to evolve a migratory lifestyle. Lastly, latitudinal migration includes the many birds that breed in the Northern Hemisphere, that fly south for the winter, with bird migration in the Southern Hemisphere being a mirror image of that in the North. While commonest in birds seasonal north-south movements do occur in other animal groups, particularly mammals (caribou, polar bears, and certain bats) and insects (various butterflies, moths, and dragonflies). In oceans, the best known latitudinal migrants are whales, seals, and walruses.
The Urge to Breed
There comes a time when animals must find a mate, and hunt for a safe place for their precious eggs or offspring to develop. Reproduction is a critical stage in the life history of any living creature, one which compels many species to make a special migration. Mammals often have a social structure based on sexual segregation, where the adult males live apart in bachelor groups, or as loners. This system which is typical of herbivores, forces males in breeding condition to search for females in heat. Males may converge on traditional courtship grounds, for example, deer, antelope, and wild sheep assemble at the annual rut. Alternatively, in species such as elephants and rhinos, each adult male embarks on a solitary, testosterone-powered expedition to locate females.
Amphibians are the only terrestrial vertebrates alive today that develop from a larval stage. Their larvae are aquatic, equipped with water-breathing gills instead of lungs. Inevitably, therefore, the adult phase of an amphibian’s life-cycle is punctuated by a series of return migrations to fresh water to breed. Numerous frogs, toads, and salamanders carry out spawning migrations, which vary enormously in both length and choice of breeding habitat. For some species, any nearby puddle will suffice, whereas the rest may trek to a pond or march up to a few miles away, and revisit the same locality year after year. Amphibians migrate on wet nights, to keep their permeable skin damp. The clue for their departure is the first cloudburst of the rainy season or a sharp rise in temperature in spring.
Among reptiles, the principal type of migration is to and from traditional egg-laying sites, under taken by fresh water and sea turtles. Like their ancestors, these turtles produce soft-shelled eggs with the texture of parchment, which obligates them to lay on dry land. They need beaches with just the right gradient and sand, hauling ashore to dig pits that act as efficient incubators. Since suitable beaches are few and far between, the turtles tend to nest communally in the best spots. Reptiles are not alone in making repeat migrations to long-established nest sites. Seals, sea lions, and walruses have also retained their ancestral link with the land unlike whales and dolphins, they have not solved the problem of giving birth at sea. Sheltered beaches, rocky coasts, and ice floes serve as their nurseries. Sea birds can not rear their young at sea, either; consequently, they flock to coastal cliffs and remote oceanic islands to nest in their tens of thousands. The breeding migrations of shearwaters, albatrosses, and terns are some of the longest and most spectacular in the entire animal kingdom.
The Moon is unusually large and close compared to the moons of other planets and so has a powerful influence on life on Earth. By virtue of the tides that it causes, the Moon’s impact on marine animals is greater. Marine fish, turtles, and invertebrates all synchronize their reproductive migrations with the tides, often choosing a particular moment in the lunar cycle. For example, huge numbers of American horseshoe crabs, distant relatives of spiders, swarm across beaches at full moon during high spring tides. Timing their spawning to coincide with the year’s highest tides allows these prehistoric looking arthropods to ensure their eggs are deposited in the upper zone of the beach, comfortably beyond the reach of savaging intertidal creatures. Other tide-driven breeders include two small fish, California grunion and cupelin, which surf in on the waves en mass and fling themselves onto the wet sand to mate and lay their fertilized eggs.
Nomads and Invaders
Not all animals are in response to seasons, some lack a fixed distinction or route. Wholly unpredictable, these movements include itinerant wandering, larger scale invasions, and spontaneous flights to escape bad weather or volcanic eruptions. Sometimes pioneering individuals encounter ideal living conditions and remain in their new territory as colonists. The ability to be flexible in movement patterns is an important aspect of coping with environments that have a patchy food supply or irregular rainfall. It is more beneficial to move from place to place according to need than to follow a pre-programmed migratory regime. Such a lifestyle is known as nomadism and is characteristic of species that live in grassland and desert habitats. The savannas of Africa tropics, for example, are home to legions of highly mobile hoofed mammals, which hop between the fertile green “islands” produced by localized rain. Their temperate counterparts are skittish herds of Mongolian gazelles and saiga antelopes, which have adapted to survive in the parched grassy steppe that covers vast swathes of Central Asia, where standing water is almost nonexistent. Interestingly, birds that undertake long latitudinal migrations are often sedentary on their nesting areas and nomadic while on their wintering grounds. For example, North American wood warblers join roving flocks of local birds in the forests of Central and South America.
An “invasion” is when a more or less sedentary population of animals is suddenly impelled to move as a result of overcrowding or food shortages. From an ecological perspective it takes place at the moment when living standards have declined so far that the situation becomes untenable and mass emigration is now the best option. Invasions are usually erratic events that drive a species beyond their usual range, and occur in a variety of birds and rodents from the far north, most famously, lemmings. They are an important phenomenon in the later cycles of many insect pests, such as locusts.
Some birds react very quickly to deteriorating weather, which forces them to move elsewhere without warning. This type of weather-induced exodus, called an escape movement, can mean the difference between life and death to a bird. The most dramatic escape movements are carried out by swifts, superbly aerobatic species with scythe-shaped wings, which trawl the sky for flying insects. Swifts are supreme fair-weather birds, totally reliant on clear skies to find food. At the first hint of an approaching low-pressure system, they circumnavigate the depression, flying over or under the front to reach the calmer air behind it. This may involve a round-trip of over 1250 miles in only a few days.
Young animals often appear to be hard-wired with a powerful wanderlust. The exploratory urge takes them away from their birth or hatching site, so that they can familiarize themselves with their neighborhood. Additional benefits of juvenile dispersal are that it helps a species to spread out through all of the available habitat, and reduces the risk of inbreeding. Juvenile dispersal is seen in many animal groups. One example is with juvenile king penguins that may swim over 600 miles from their natal colony, while young African penguins, which start life on the coasts of South Africa, frequently reach the Atlantic’s equatorial waters.
In essence migration is about being in the right place at the right time, and so migratory species need some form of in built clock. Accurate time keeping allows an animal to keep in step with changes in the outside world, and to begin and end journeys on cue. It is also essential for successful navigation.
Some examples of astonishing feats of time keeping: sea turtles and crabs return to their nesting beaches on the same few nights each year, shoals of fish appear at certain locations in the ocean on a predictable schedule, and generations of migratory birds arrive back on their breeding grounds within a couple of weeks of their species’ traditional date. In 2008, scientists announced that they had discovered “clocks” inside individual human cells. Such mechanisms most likely exist at a cellular level in animals as well. What are internal clocks? The answer is complex, and the way they work is not yet fully understood. Some types of animal behavior such as self-defense can be provoked spontaneously they do not operate according to a timetable. However, lots of other processes including feeding, sleeping, metabolism, and reproduction, are governed by strict 24-hour cycles. These cyclical patterns of activity are called circadian rhythms. They may be influenced by external cues, changing temperature or humidity, the rise and fall of the tide, or the alternation between night and day, for example, but are internally generated and instinctive. In addition, there are long-term regulatory cycles, known as circannual rhythms. These also respond to stimuli outside the body, such as gradually changing the day length, and the yearly sequence of passing seasons and are primarily internal. Circannual rhythms are most developed in species found in the world’s temperate regions, especially near the poles, where the annual change in day length is greatest. Lastly, circadian and circannual rhythms work together to create perfectly calibrated extremely efficient clocks. They are unique to each species, having evolved to suit its particular lifestyle and environment. It follows that these deep-seated mechanisms are fundamental to the ability of migratory species to plan and coordinate their journeys successfully. One species of birds, the black-tailed godwit, has developed a system for synchronizing its migrations so that both the male and female arrive at their breeding grounds at the same time each year, even though they spend the winter apart and travel to and from their wintering areas separately. These migrants have developed an amazing capacity to synchronize their migrations, typically showing up within three days of each other, despite being separated for months and flying thousands of miles. To date, no one knows for sure how the birds do this. Lastly, the majority of animals have a “pace maker” in overall control of their circadian and circannual rhythms. In mammals, the pacemaker is a part of the brain known as the S.C.N. (short for suprachiasmatic nucleus). The hormone melatonin also has a vital role to play. Melatonin is secreted by the pineal gland, but only at night, because daylight blocks its production. Therefore, length determines how much of it the body makes, and the melatonin level helps regulate daily and seasonal activity. The pineal gland appears to act as the master pacemaker in fish, reptiles, and amphibians. Some migratory animals have to suspend their normal circadian rhythms temporarily. For example, species that travel to the Artic or Antarctica for the polar summer encounter near-constant daylight, which means their circadian rhythms based on the daily light-dark cycle can not function. Animals such as caribou therefore become “non-rhythmic” during the polar midsummer.
Surviving the Journey
Migration is unforgiving on those who take part. It can be a relentless struggle and often puts animals under enormous stress pushing their metabolism and other body processes to the limit. The odds might seem to be stacked heavily against ever reaching the destination but the reality is that generally most migrants do arrive unscathed, thanks to a suite of physical and behavioral adaptations.
Some species are spectacularly ill-suited to a migratory life. Big cats for instance do not make good migrants since they produce young that are helpless for many months. Size can be an important factor to: the majority of small land animals simply can not afford the energetic cost of migration. Most rodents lack the endurance to cope with regular long-distance travel; a 3.5-oz rodent would use about 25 times more energy per unit of body mass than a 450-pound antelope. By contrast, the entire life history of other animals may be geared toward migration. Antelope and gazelle calves are on their feet only minutes after birth, and due to their disproportionately long legs and extremely high fat content of their mother’s milk, they can keep up with the rest of the herd within days. The young of tundra-nesting shorebirds develop so rapidly that they can embark on their maiden southbound migration from the Artic when only two months old. And sea turtle hatchlings are already proficient swimmers and head straight for the safety of deep water.
Not all migrants begin to migrate when still a baby-many have to ensure that they are in a fit state first. In adult birds, this preparation includes a molt, the timing of which is controlled by hormones and the bird’s inbuilt circannual rhythm. Replacing old and worn plumage is crucial, because flight efficiency depends on the condition of the wing feathers.
Migratory animals often feed intensively prior to departure. The aim of this gluttonous behavior, known as hyperhagia, is to boost fat reserves to use as fuel. Hyperhagia is switched on automatically by an internal circannual rhythm and is seen in animals as varied as monarch butterflies, caribou, and baleen whales; in insects, it can lead to a 30 percent increase in body weight. The migrants-to-be do not only eat more, but also look for particularly high-energy foods. In temperate latitudes, insectivous birds such as warblers and thrushes shift to a sugary diet of fruit late summer and autumn to lay down a thick layer of fat before they set off.
Finally, animals may go through a radical physical transformation. Birds develop larger, more powerful breast muscles and shrink nonessential organs accordingly (to avoid excessive weight, which would hamper efficient flight). Some insects do much the same, for example, the generation of monarch butterflies migrating south through North America in the fall has no sexual organs, these develop the following spring. And desert locusts grow longer wings and look like a completely different creature by the time they take off.
Managing the Risk
Over thousands of years, migratory animals have evolved many solutions to the problem of how to make their journeys less dangerous. To combat the ever present threat of predators, migrants frequently travel in groups or at certain times of day or night. It is a good idea to take advantage of favorable environmental factors, such as the wind or ocean currents, and finding the right pace. There are both fast and slow migrants, every species follows its own migratory timetable suited to its strength, stamina, and fat reserves, and the distance to be covered.
Staggered migration, with stops to rest and refuel, is another common strategy. Bats visit a series of conveniently located roosts during their journey, while butterflies and moths settle on trees and buildings, to roost overnight or until a spell of bad weather has passed. Traditional locations for resting are stopovers, or staging areas. The best are used year after year, and at key periods host vast gatherings, particularly of birds. For example, approximately 45 percent of all migratory shorebirds resting in America stop to rest, or “stage”, at Cheyenne Bottoms, Kansas, in the spring. Protecting major staging areas such as Cheyenne Bottoms is therefore a conservation priority.
A Helping Hand
The forces of nature offer welcome assistance to tired migrants. Insects and land birds ride tailwinds and circle upward in thermals, seabirds catch the updrafts created by waves, and turtles and fish are pulled along by ocean currents. As a result, the Earth’s prevailing winds and currents exert a powerful influence on the timing and direction of migratory journeys.
The oceans are never placid like lakes, even on the most halcyon days. Although the sea may look calm at certain times, currents and upwellings surge below the surface. In addition, the tidal cycle has a profound impact on marine life, affecting water circulation far from land, especially during the largest “spring” tides that occur twice each month. So it comes as no surprise that pelagic species, inhabitants of the open ocean, should make full use of their constantly moving, three-dimensional habitat, by ascending and descending through the water column to find a powerful flow that will propel them in the right direction. There is an added bonus to swimming in fast currents: they are often laden with food.
Many currents follow definite routes that can be drawn on a map, and migratory animals develop a close relationship with them. In summer leatherback turtles pick up the warm Gulfstream to glide across the North Atlantic to the coasts of northwest Europe, which teem with their favorite jellyfish prey, while the movements of manta rays and whole sharks show a clear correlation with warm currents throughout the tropics. The larvae of countless pelogic fish, mollusks, and crustaceans rely on currents to drift away from their spawning areas.
Just as the marine environment is in permanent motion, so too is the planet’s air. Atmospheric conditions can change rapidly, which means birds and other aerial migrants have to time their departure carefully; sometimes a delay of only a few hours may be disastrous. The ideal scenario for migration is a sustained tailwind and cloudless skies. Lower than average temperatures are also a boon, because the cool air prevents hard-working pectoral muscles from overheating. This is one reason why some birds migrate at night, particularly when flying across deserts where the punishing midday heat could prove fatal.
Flying migrants avoid setting off on windless days, as the still air obliges them to use more precious energy. No birds are more dependent on the wind than albatrosses, most species of which use the wind to help them soar and glide over the tempestuous Southern Ocean on characteristically stiff wings. Faced with a becalmed sea, these majestic ocean wanderers can not stay airborne and are reduced to bobbing around on the surface like rubber ducks.
Having found a thermal, these species ascend in lazy spirals, seldom bothering to beat their wings. Once they arrive at the top of the up current, they glide out and away, gradually losing altitude until they locate another thermal and can repeat the procedure. “Thermal hopping” is an amazingly efficient made-of travel, but since thermals do not form over water, at night, or in cold weather, it limits where and when soaring migrants are able to move.
Most migrants are creatures of habit that have a tried and tested route map. Rarely is this a straight forward line drawn directly between two points. Migratory journeys are shaped by the physical geography of the land and ocean, so looping routes and diversions around major barriers are common, and the outward and return legs may be different.
Animal migration generally advances along a broad front, which can be hundreds of miles wide. This front consists of many separate parallel streams of migrants simultaneously following the same bearing. If the progress of every individual taking part in the migration was plotted on a map at regular intervals, the resulting pattern would therefore resemble a wave, sweeping forward in a long line. A huge variety of animals from hoofed grazers to waterfowl, small songbirds, bats, butterflies, dragonflies, and land crabs, migrate in this fashion.
However, some species use a much more restricted migratory artery, known as a narrow front. This style of migration is found mainly in large land birds, such as storks, cranes, and birds of prey. It is also typical of coast-hugging ocean migrants, including gray and right whales, which follow the shores, seldom straying from the shallow waters of the continental shelf.
The shortest route is not necessarily the easiest or safest. Certain physical features, called leading lines, encourage migrants to adapt a particular path, regardless of whether their journey is extended. Leading lines include rivers, streams, lake shores, valleys, mountains, and coasts. They are used by terrestrial animals great and small, and by a wide range of aerial migrants too. Often they are in themselves natural barriers to progress, for instance, the Rockies, Appalachians, and Andes present a massive hindrance to west-east migration, yet enormous numbers of migrating insects and birds funnel down their flanks in a north-south direction.
Oceans also possess leading lines, which are equally important to those on land, albeit less obvious. Beluga whales lead north to their summer feeding grounds in the Artic by swimming up “leads”, narrow cracks in the sea ice that serve as convenient expressways. Below the surface of the world’s oceans, turtles and fish follow submerged mountain chains and move along the steep drop-off on the seaward side of the coral reefs. Sharks appear to be able to detect and follow “roads” of migration across the seafloor.
Waifs and Strays
Sometimes migratory journeys go so badly awry and animals head completely off target. Birds, bats, and insects can get into difficulties in low cloud and heavy rain, which severely hampers their ability to orientate. In the worst-case scenario they may end up far from their species’ normal range, occasionally making landfall on ships and oil rigs in mid-ocean, or even on the wrong continent. Cross winds are another potential problem: a gentle side breeze might seem harmless at first, but if it strengthens during the flight, an airborne migrant will unwittingly draft further and further off course.
In general, older, more experienced individuals are more likely to recognize and be able to compensate for these differences. The vast majority of hopelessly lost migrants, which biologists refer to as vagrants, are juveniles on their first outing. Perhaps the most tragic case of migratory inexperience is when sea turtle hatchlings become confused by brightly illuminated beachfront developments, mistaking them for the dim glow over the ocean, with the result that the baby reptiles crawl inland instead of toward the breaking surf.
Animals have evolved highly efficient direction-finding systems, in which visual clues often play a central role. Migrants look for familiar geographical features, orientate by the Sun, and interpret the movements of Stars in the night sky, enabling them to steer the right course over great distances, with pin point accuracy time after time.
Looking for Landmarks
Many migrants scan the landscape for short range navigation, usually to find their precise target during the final stage of their journey. High-flying birds obtain a panoramic vista of the ground below, so look out for reference points such as rivers, and coats, building up a picture of their surroundings that they remember from one migration to the next. The horizon is in itself a helpful cue. Research has shown that homing pigeons seldom approach their loft along a straight compass bearing, but are guided by the twists and turns of major features in the area, including artificial ones such as roads and power lines. Even birds that migrate at night are helped by landmarks, from the glint of moonlight on water to the bright lights of a town.
Marine animals also are thought to navigate using knowledge of local topography. For example, seals and whales can probably recognize features of the seafloor. Some marine biologists have suggested that sea turtle hatchlings “imprint” on unique characteristics of their natal beach, and that year’s later adult female’s recall this information to help them locate the same stretch of sand.
The Sun is an ideal visual reference, everyday it rises in the west and sets in the east, and at noon in the northern hemisphere it lies due south. To use the Sun as a basic compass, it is necessary to tell the time, but as previously discussed, all migrants have internal clocks. Numerous experiments have demonstrated the existence of a Sun compass in animals. One of the most famous was conducted in 1950 by German scientist Gustav Kramer, who manipulated the apparent angle of the Sun to see what effect it had on European starlings. When Kramer used mirrors to bend the Sun’s rays through 90 degrees, the starlings changed their preferred flight direction accordingly. The same test has been carried out on monarch butterflies, with similar results. Other experiments have proved that birds can orientate themselves by referring to shadows cast on the ground rather than observing the Sun directly, and that they have a built in compensation ratio to allow for the Earth’s rotation.
Animals as diverse as insects, birds, and crabs have another useful skill: they are able to detect polarized light, which means they can orientate even on cloudy days when the Sun is hidden. Polarized light patters are formed when light is scattered by airborne particles. Since the Sun’s position shifts throughout the day, the overall pattern of polarized light in the sky also changes.
Few birds that travel at night seem to orientate by the position of the Moon. Instead, they observe changing star patterns. The crucial factor is the rotation of the night sky about a fixed point, which is the northern hemisphere is the Pole Star. Nocturnal migrants have no need to see individual stars or constellations so long as they are able to make out the center of rotation.
The American biologist Stephen Emlen investigated the nature of the star compass using captive indigo buntings in a planetarium. The birds were placed on an ink pad at the bottom of a paper cone so that the resulting pattern of inky footprints indicated their preferred direction of travel. When Stars in the planetarium were obscured, the buntings became confused, hopping aimlessly. But when a real sky was projected, they soon located the Pole Star and used it to find north, the direction of their spring migration from Central to North America. When the entire sky was rotated, the birds still managed to head north, proving that movement of stars rather than their position is the key.
How bats orientate is less well known, although big brown bats and certain other species have been shown to use the lingering post-sunset glow to home in on their caves. The same technique might be employed by long-range migrants such as North American hoary and gray bats and European noctules.
Every migratory animal has a range of orientation mechanisms, many beyond the realm of human perception. Some species find their way by smell, taste, or sound. Others analyze subtle changes in water quality. Most remarkable of all is the ability to orientate by sensing tiny variations in the Earth’s magnetic field.
The Earth behaves like a great giant magnet. It has a doughnut-shaped magnetic field, made up of elliptical force lines running between the magnetic north and south poles. The Earth’s magnetism was long suspected to assist migratory animals, but the existence of a magnetic compass was not proved experimentally until the 1960’s, when German researchers Friedrich W. Merkel and Wolfgang Woltschko placed European robins in cages surrounded by electrical coils and then subjected them to changes in the force field around them. If the field was altered so that magnetic north appeared to lie in a different direction, the robins modified their preferred flight path. Further tests showed that the robins could feel the changing angle of the magnetic force lines, enabling them to establish which part of the Earth’s surface they were flying over.
Birds are not alone in possessing a magnetic compass. Butterflies, salamanders, newts, lobsters, bats, whales, turtles, and sharks all have one. In fact, magnetic sensitivity is widespread in the animal kingdom. How the compass works is still not understood, although the active ingredient is believed to be magnetite. In 1979, a team of scientists led by Dr. Charles Walcott sensationally discovered particles of this magnetic substance in the head of a homing pigeon, and tiny quantities of magnetite have since been found in many other migratory animals, most recently in the thorax of monarch butterflies.
The advantage of a magnetic compass is that it is unaffected by cloudy weather or changing day length. It can fail during violent electrical storms and sun spot activity. There seems to be a link between sunspots and mass beaching’s of sperm whales, which suggests that their magnetic compass had been disabled.
The miracle of migration is that it is instinctive. A migratory animal’s brain is hard-wired with a preset “program” that tells it to proceed in a specific direction in a certain way at a given moment in time. Naturally, there are exceptions, but in general most species are born migrants, inheriting a full set of instructions from their parents that will in time enable them to complete the task.
It used to be assumed that migratory journeys were essentially the product of practice and experience. The problem with this theory was that it could not adequately explain how young animals left by their parents at an early age manage to migrate unassisted. For instance, most species of bird carry out their maiden migration with no parental guidance. The same is true of a host of other animals, many of which never meet their parents.
We have now grasped that migration is genetically predisposed behavior. It is controlled by complex inner drives passed down from generation to generation, though this does not preclude individual migrants from getting better due to their life experiences; as might be expected, older migrants are indeed the most accurate navigators. Regardless of whether instinct or learning is more important to migration, some of the most exciting research is focusing on the type of inbuilt map that migrants might use.
True navigation, by definition ought to require a mental map of some kind, because for it to be possible an animal would have to possess a benchmark against which it is able to establish its current position relative to its ultimate destination. Some scientists propose that there is a different navigational system yet to be discovered. For example, animals may resort to a “gradient” system, whereby they repeatedly compare two changing features of the planet’s surface against each other.
One thing is certain, if a migrant were to rely on a mental map to reach its goal, it would have to store a vast amount of information in its head. To check its progress during a journey, it would need to refer to this data bank repeatedly using the full range of orientation clues available, both visible and invisible. Whether this is possible is still unknown.
Learning the Ropes
The young of some migratory animals remain with their mother, or both parents, for a considerable time, building up knowledge of a large area and undertaking an entire migration cycle in their company before finally becoming independent. This behavior is seen in geese, swans, and cranes, which travel in close-knit family units comprising both parents, and it is also found in various whales and hoofed mammals, whose offspring migrate with their mother while still suckling. Since the new generation has the opportunity to acquire the established migration pattern from the previous one, these animals are exceptions to the rule, that a well-developed migratory capability is present at birth.
Genetics and Migration
European cuckoos provide a classic case study for researchers investigating the genetic basis of migration. These dove-sized birds are brood parasites: that is, the females lay their eggs in the nests of other birds. Yet despite being raised by foster parents belonging to a different species, the young cuckoos duly set off at the end of the summer to migrate southward along much the same routes used by their true parents a month or two earlier, joining the rest of their species on the wintering grounds in East Africa and Southeast Asia. In other words, juvenile cuckoos simply “know” what to do. They must be following a migratory schedule inherited from their parents, complete with a preferred direction of travel.
Evolution of Migration
Migration is a dynamic, constantly evolving process. Though individual migrants adhere to a predictable route and routine, their species’ migration patterns will, over time, alter in response to changing threats and opportunities in the world around them. Migration is but one example of the relentless adaptation of species to their environment as they compete for living space and resources.
The Earth has been in a state of flux throughout geological history, with profound consequences for animal and plant life. Over millions of years, the realignment of continents and the appearance of new coastlines, land bridges, and mountain chains have made animals continually modify their migration routes. These forces are felt by marine species too. For example, the marathon migrations undertaken by humpback and gray whales were probably far shorter in the past, since when continental drift has pushed their cold-water feeding and warm-water breeding areas further and further apart.
Arguably the most important factor influencing migration has been the cycle of ice ages, which has driven the habitat that each species needs back and forth across the surface of the planet. Many of the long-haul animal migrations seen today have their origins in the global warming that brought the last ice age to an end, approximately 10,000 years ago. As the Northern Hemisphere’s ice sheet started to recede toward the Artic, so did the tundra zone that formed its southern boundary. Species that visited the tundra to breed, such as caribou, sandpipers, swans, and geese, found that the journey to and from their winter quarters, in milder climes to the south, became slightly longer, with every generation.
Migratory behavior is not evenly spread throughout the biological world, it tends to evolve in certain biomes and around groups but not in others. For example, true migration is rare among the primates, despite there being about 230 living species. The same can be said for parrots; of the 350 species known, few carry out long seasonal movements, and only two ever cross the sea on a regular basis. This is mainly because both primates and parrots have a strong preference for tropical forests, where the constant high temperatures and abundant rainfall ensure luxuriant plant growth all year, and there is little incentive to travel.
By contrast, tropical grasslands experience alternating wet and dry seasons, with dramatic climate extremes. As a result, vegetation growth occurs in spurts, between which the rain-starved landscape turns golden brown and food becomes difficult to find. The dominant groups found in the savanna biome are herbivores, particularly even-toed ungulates, and it is no coincidence that most members of this great mammal order lead highly migratory or nomadic lifestyles.
Genetic Basis of Migration / Natural Selection
The genetic basis for migration means that, like any inherited trait, it develops by the process of natural selection. Each species’ in-built migratory “program” adapts gradually, over successive generations, to improve the chances of survival. The change is usually almost imperceptible to us. Sometimes, however, it happens fast enough to be measurable within a human lifetime.
One notable case is the migration of the blackcap, a small, insect-eating songbird that breed in woodland in Europe and western Asia, wintering in Africa, south of the Sahara. Since the 1970’s increasing numbers of blackcaps have stayed behind to spend the winter in northern Europe, no doubt in response to warmer weather. This non migratory behavior probably started by accident, when a few individuals failed to migrate in the normal way, but it is now entrenched in a subpopulation of northern blackcaps because it saves them the effort of migration, thus enhancing their survival prospects. Eventually the populations could evolve into separate species.
Our understanding of migration has come a long way. Now the journeys of migrants as small and fragile as dragonflies can be tracked in great detail using sophisticated transmitters and data loggers, which new analytic techniques let scientists delve into the chemical make-up of migrants themselves. Until the early 20th century, most observations of migratory patterns in animals came to light through activities such as whaling and big game hunting, or by gathering specimens for display in natural history collections.
But in the 1940s and 50s, as field identification skills and quality of optical equipment improved, naturalists began systematic studies of animals actively migrating. A network of observations was established at islands, headlands, and other migration hotspots to log the passage of birds. Today, we live in the age of the online census: sightings uploaded by dedicated armies of observers help to build, in real time, a detailed picture of animal migration in its myriad forms. To keep up with active migration, researchers take to the skies in everything from microlites, to hot air balloons, and light aircraft. Aerial surveys are useful for courting elusive, wider ranging species, particularly grassland herbivores and large ocean-going voyagers such as blue whales, white sharks, and leatherback turtles. In order to survey a representative area, spotter planes zigzag along fixed transects at a height of about 500 feet.
One of the simplest methods for studying migration is to mark individual migrants so that they can be recognized. It is then possible to get an idea of their movements by drawing a straight line between points where they are seen. This approach dates from 1899, when Danish teacher Hans Christian Montensen visited European starling nests and gave each nestling an aluminum leg band engraved with a unique serial number and return address. If anyone came across one of his birds, they could send back details of its new location and the date it was found.
Since 1899, more than 200 million birds are estimated to have been banded around the world, of which only a fraction have ever been “recovered”. However, a recovery rate of as low as 1 in 300, the average for small birds, still gives a valuable insight into where migrants go. Alternatives to banding include labeling with dye and attaching plastic tags to the neck or back, procedures that are equally effective with mammals. There are special tags designed for the flippers of sea turtles. Even insects have been marked successfully, for example, by painting the wings of monarch butterflies or sticking tiny labels onto them.
Radar and Satellite Tracking
The rapid development of radar after the Second World War enabled actual migratory journeys to be plotted for the first time. Though radar operators were initially puzzled by waves of interference on their screens, referring to them as “angels”, they soon realized that these patterns were caused by streams of migrating birds. Modern radar is powerful enough to pinpoint the height, speed, and wing beats of individual birds and bats, while its aquatic equivalent, sonar, can detect shoals of fish moving underwater.
In recent decades, research into animal migration has been revolutionized by satellite tracking, also known as satellite telemetry. The migrants being studied are fitted with tracking devices called platform transmitter terminals (PTTS), which beam signals to orbiting satellites according to a preset cycle, and those in turn relay the information back to coumpters on the ground. Since the 1990’s, transmitters have become progressively smaller and lighter, with a longer battery life, and stronger signal (hense range), allowing the progress of migrants to be followed for months. The latest transmitters come in many types, including collars, darts, anklets, and miniature units glued to the thorax of insects, and give updates about an animal’s body processes and environment as well as its movements.
In addition, to tracking devices, there are long-term data-storage instruments, generally used with fish. Often these are surgically implanted into fish, but the drawback with internal tags is that to retrieve them the subjects need to be caught again and then dissected. An alternative is to use pop-up archival tags (PATS), which are programmed to detach at a preset time and float to the surface to upload their stored data via satellite, meaning that it is not necessary to recover the gadget itself.
Migration Over Land
Migration In Water
Migration By Air