Animal migration is a wondrous thing. Driven by seasonal changes, an incredible array of organisms fly, creep, or run on their way to or from richer spots for wintering, feeding or breeding. ~ American naturalist Joel Greenberg
Migration is a ubiquitous feature of animal life, found in all major groups: insects, crustaceans, fish, amphibians, reptiles, birds, and mammals.
Migration may be triggered by local climate, food availability, season, to escape predators, or for breeding. Plants migrate when provoked by climate, albeit much more slowly than animals. Birds began migrating to escape the harsh winters in the northern hemisphere.
Animals that don’t migrate tend to be tropical, such as parrots and primates. The exception are herbivores, who rely upon rainfall patterns in the grasslands. The great migration in the Serengeti is a trek of 70,000 km2 for huge herds of wildebeest, antelope, gazelle, and zebra.
Migration is a broad term. The ungulate trek about the Serengeti illustrates foraging commuting: following food supply in a circuitous route. Truer to the concept that migration implies is area-to-area trekking (destination migration), commonly for seasonal climate reasons and/or for breeding.
Migration distances vary greatly. The North American blue grouse winters in a mountain pine forest. Come spring the grouse walks down 300 meters into deciduous woodland, where it feeds on leaves and seeds. Similarly, tropical hummingbirds may seasonally go up or down a mountain a few hundred meters.
The Arctic tern is at the other extreme. This Sun-loving bird breeds in the Arctic regions during the summer there, then flies to Antarctic for the southern summer. Arctic terns stay airborne for days and nights at a stretch. Annual travel is 70–80 thousand km per year.
A bird’s migratory habits reflect seasonal food supply. Residents pretty much have enough to eat year-round.
Nomads, such as budgies and crossbills, suffer less reliability in food supply, but have low seasonal variation. Nomadic birds, like other such animals, move when food runs low, but generally to a selfsame habitat.
Irruptive migrants, like hawks and owls, migrate en masse when food runs low. Obligate migrants, such as thrushes and warblers, trek south for the winter from their northern summer home.
Long-distance migration is a high-risk enterprise: only about half of all migrants reach their destinations. By contrast, ~85% of non-migratory birds make it through the winter. As an adaptive compensation, many migratory birds have higher reproductive rates than those that stay put.
Owing to the changing nature of their environment, most fish migrate. Many species, such as Pacific salmon, migrate early in life, from fresh to sea water and back again to lay their eggs. Even those that live an enclosed body of water, like a lake, may breed in one location and feed in another.
Only 1 group of fish – freshwater eels – start life in the ocean and spend their adulthood in fresh water. Unlike other fish, eels can cut across fields if need be, traveling for kilometers on land like snakes, as long as the ground is moist enough to keep them from drying out.
European eels travel from the tropical Atlantic to the same European rivers their parents inhabited. When it is time to spawn, they head back out to sea.
Animals that migrate in groups learn routes from experienced trekkers. Migration routes are a cultural folkway.
Environmental cues and internal clockwork signal when it’s time to travel. Many migratory animals, especially long-distance travelers, practice hyperphagia: intensive feeding, particularly of energy-rich foods. Insects might bulk up by 30%. Whales sometime double their weight. In temperate latitudes, insectivorous birds, such as thrushes and warblers, switch to high-fructose fruit to lay down a layer of fat before setting off.
Animals may also undergo radical physical changes as part of migration preparation. Desert locusts grow longer wings. The nonessential organs of migratory birds shrink while their breast muscles enlarge and grow more powerful.
A small resident bird may have 3–5% body fat. One that is a short-range migrant – annual treks of several hundred kilometers – may bulk up with fat reserves that account for 13–25% of body mass. A transcontinental flier may take off with half its weight in fat.
The reserves are essential. The hourly cost of flight is 0.91% of a bird’s body weight.
Half of the breeding birds of North America are seasonal migrants: flying south to winter in Mexico, or Central or South America, and returning in the spring. In an example of convergent evolution, migration arose in numerous birds from resident species.
Many songbirds undergo seasonal migrations. Although these birds are typically diurnal, migratory flight is done at night. The drastic lack of sleep does not down their physiological or cognitive performance during the migratory period (though it does otherwise).
Most songbirds that seasonally migrate in the New World take a continental route so they may land for respite if necessary. The forest-dwelling blackpoll warbler instead flies up to 2,800 km nonstop – between eastern Canada and South America – over the Atlantic Ocean in 2–3 days.
It’s a fly-or-die journey. ~ Canadian ecologist Ryan Norris
A water landing would be fatal. The tiny blackpoll warbler weighs but 12 grams: about as heavy as a cheap ballpoint pen. But it is one determined bird.
Blackpoll warblers fatten up for the flight: doubling their normal body mass. A blackpoll warbler knows exactly where it is going: returning to same place as the previous breeding season. The trip takes its toll. Only about half of blackpoll warblers make it back the next year.
For birds, small or large, body size is no barrier to migration. Some of the biggest birds – cranes, storks, eagles – migrate by riding thermals.
Once the morning Sun has heated the ground, birds rise on the ascending spirals of warm air, then glide. They stop for the night in the mid to late afternoon, as the thermals dissipate for the day.
Such soaring takes little energy for these birds, which have fattened themselves beforehand and eat little en route. Resting hours are spent roosting.
For thermal riders, migration is a rather leisurely affair: a tour of the continent(s), for thermals only form over land. If a gliding migrator wants to cross continents, it will follow whatever land corridors may be had to cross the water where it is most narrow.
Big birds traveling between Europe or Asia to Africa either fly over the Middle East, with only the Suez Canal to flap over, or take the westerly route over the Straits of Gibraltar, a mere 14 km across. Similarly, the Isthmus of Panama is the bridge between North and South America.
Following such specific routes leads to considerable traffic buildup: hundreds of thousands to millions of single-species travelers might be a norm, at least before humans grossly reduced their numbers.
Not all flappers are not so constricted. Osprey, a large raptor, migrate over the Baltic and Mediterranean Seas.
Smaller birds power their way along the straightest route, land, or sea, as long as they have the strength to need not stop over a long stretch of water.
Many long-distance migration flocks fly in V formations that are energy efficient. Birds in these formations account for complex fluid dynamics between ambient air currents and the flapping of their fellows’ wings ahead of them to minimize effort. It is a physics equation which we can hardly imagine that these birds continuously calculate.
V-formation flight is an exercise in reciprocal altruism. Being a lead bird is taxing. So, birds regularly swap, for equity’s sake.
Birds match the time they spend in the wake of each other by frequent pairwise switches of the leading position. ~ Austrian zoologist Bernhard Voelkl et al
While some animals, including Pacific salmon and lemon sharks, innately remember their natal home, many learn migration routes through the experience of traveling with their elders. This is true of most animals that migrate in groups, including flocks of birds.
For all its appearance of hazard, migration is simply a path for survival. Many animals migrate in groups to mitigate risk. Others, such as bats, stagger migration, with stopovers for a brief respite. In contrast, seafaring birds may push themselves to their physical limits. The Arctic tern is but one example.
Whooper swans fly from Iceland to their winter home in Britain, traveling 65–85 kph for 12–13 hours straight; quite a feat for an 8–14 kg bird.
Migration is often most impressive considering the size of creature involved. Tiny ruby-throated hummingbirds which weigh a penny fly nonstop across the Gulf of Mexico twice a year: an 850-kilometer trip.
Before they embark, ruby-throated hummingbirds bulk up by binge eating: sucking up every drop of nectar, scarfing down every insect and spider they can snag. By the time the hummingbird takes off, it may be double its normal weight.
Migration may be conditional or optional. Some European blackbirds migrate in the autumn, while others overwinter on the breeding grounds.
Birds make a migration choice suiting their social standing. Younger birds head off in the fall as dominance contests are becoming a regular event. Older birds stay put to hold onto their turf.
The common poorwill is a nocturnal bird native to western North America. Part of the population migrates to warmer climes in the winter, leaving others behind to hibernate in rock nooks and crannies.
The common poorwill is the only bird known to hibernate. Other animals, including frogs, toads, rodents, bats, and bears, cope with inhospitable winters by tucking into an extended torpor.
Some species have populations with a mixture of migration and sitting tight. Marine bristle worms, such as the palolo, take such hedging to the extreme.
A palolo splits into 2. The sexually mature tail end (epitoke) takes off, while the sexually immature head (atoke) lives on where it is. Synchronized with the lunar cycle, epitokes rise to the ocean surface en masse, bursting open to release sperm and eggs, thus restarting the worms’ life cycle.
Before animal migration was understood, various explanations arose for the sudden disappearances and arrivals of animals. Aristotle proposed bird transmutation: that robins turned into redstarts with the coming of summer. Swallows supposedly hibernated in tree holes, torpid and featherless.
In medieval European bestiaries, the barnacle goose either grew like fruit on trees, or emerged from goose barnacles on driftwood. The barnacle fiction was favored by Catholics, as it allowed the geese to be considered fish, and hence could be served for supper on Fridays and during lent.
Hibernation continued to account for disappearances. Swallows were drawn up in fishing nets from the bottom of lakes where they wintered, so the fish story went.
Though hibernation was written off as early as 1251 by English Benedictine monk Matthew Paris, a rational conceptualization was slow to gain a popular grip.
Even into the 1800s, hibernation haunted as explanatory. Otherwise, migration was conceived as an endless circling of the globe, or even to the Moon and back.
Monarch Butterfly Migration
These butterflies are little more than 8 months old and have traveled thousands of kilometers over their lifetime. ~ American biologist Nathan Miller
Monarch butterflies winter in the Oyamel fir forest in central Mexico, huddling by the tens of millions, high in the mountains. It’s cold there, but not often freezing. Snowstorms do strike the mountains sometimes, killing as many as 2 million butterflies overnight.
The lowlands would avoid the frigid risk, but the warm aridity would quickly sap monarch energy reserves; better cool and moist.
The Oyamel fir forest provides monarchs freeze protection because of the dense tree canopy. Even logging that only thins the canopy removes the frost protection.
From Oyamel, adult monarchs head north to lay their eggs on milkweed plants in the United States, or even as far as southern Canada.
In a complex process incorporating temporal and spatial information, monarchs navigate via Sun compass orientation. The Sun is not all. The monarch navigation system includes magnetoreceptors in their antennae, producing a mental magnetic field map which integrates with visual information.
Though family get-togethers are fun, migration is not a necessity. Monarchs are becoming more common in Bermuda, owing to the popularity of milkweed planted in flower gardens. Bermuda-born monarchs reside year-round, owing to mild climate.
Migratory monarch numbers dwindled precipitously in the early 21st century, as deforestation chopped down their winter lodging, fields of milkweed were converted to cropland and suburbs, and the climate proved increasingly adverse. There were 10 times as many monarch butterflies in 1994 as there were 2014: a 90% decline in 20 years. Migratory monarchs will soon be gone thanks to human interventions.
To maximize breeding opportunities, some dragonfly species migrate. Like the monarch butterfly, swarms of green darner dragonflies migrate between the northern United States and southern Mexico each spring and fall.
The globe skimmer dragonfly lives up to its name by trekking between Africa and India across the Arabian Sea; a flight which may exceed 16,000 km. To ease their passage, globe skimmer swarms prefer moist winds. The dragonflies save energy by riding thermals.
Dung beetle life is fiercely competitive. After sniffing out and rolling a ball of nutritious dung, a beetle must race home with it or risk the ball being stolen by other beetles.
A straight line across the open plains is easy enough during the day, where landmarks guide the way. The Moon guides at night, unless the Moon is new, in which case there is nothing to do but use the stream of stars in the night sky called the Milky Way. Dung ballers are star gazers that roll home.
Dung beetles can orient to polarized light. ~ Swedish zoologist Marie Dacke et al
On a hot day, when the sand is blistering, a dung beetle climbs up upon his ball for a cooling reprieve. These 6-second siestas let a dung beetle stay on the job when the heat is on.
Dung beetles are ecological benefactors. They accelerate nutrient cycling, improve soil structure, and eliminate mounds that might serve as breeding grounds for animal pests such as flies.
Migration is at once complex and mysterious. ~ English zoologist Ben Hoare
Animals migrate using a variety of senses and means, including visible clues, such as landmarks, and star patterns for nocturnal travelers; the scent of water quality or soil; the changing taste of prey along the way; and familiar local sounds.
During migration, various sensory input is continually coordinated and linked with elaborate mental maps to determine location and heading, with which to optimize getting to the intended destination, all the while factoring in local conditions while traveling, and adjusting accordingly. The currents or winds must be constantly considered. To call long-distance migration “complex” is a gross understatement.
Birds adjust their route based on their condition. The red-eyed vireo – a small songbird – migrates over the Gulf of Mexico only if carrying ample energy reserves. A low-fat bird will take the longer overland route, rimming the Gulf on the way south.
Besides sight, sound, taste, and smell, many animals – from butterflies, salamanders, newts, lobsters, turtles, sharks, bats, and whales – have an even more fundamental planetary force for a guide.
Magnetic Field Detection
Many migrants home to very precise locations. ~ American zoologist Hugh Dingle
Animals that migrate long distances, from insects to mammals, partly do so by magnetic-field detection: sensing Earth’s magnetic flux.
Cryptochrome on the Wing
Migratory insects, birds, and other animals employ the quantum property of spin to literally see Earth’s magnetic field, using cryptochrome, a photosensitive protein, in their eyes. Cryptochrome is found in the nerve layers of many animals’ eyes.
When cryptochrome interacts with a specific wavelength of blue-green light it triggers a cascade of electron transfers like that of photosynthesis, using the quantum wavelike properties of photonic energy. The photons in these transactions dance coherently via entanglement: in-phase harmonic waves which move, tip, and spin synchronously.
Normally, the electrons in cryptochrome exist in pairs. Light energy rips the electrons apart, leaving 1 electron on the original molecule while shooting the other off to another molecule. The result is 2 charged molecules (ions).
Initially, these formerly paired electrons spin in opposite directions. The spins change in the presence of an external magnetic field, altering their orientation relative to each other in a simultaneous synchronicity. The veering spins create a biochemical reaction, letting birds and monarch butterflies perceive Earth’s magnetic lines as patterns of light and color, superimposed on their view of terrain.
How such coherence can be maintained in such highly fluctuating organic systems is not known. Yet measurements of the wavelike states in photosynthetic bacteria demonstrate coherence that lasts up to 2/3rds of a billionth of a second. On the timescale for molecular events, that is an eternity.
Hatching from an egg on a beach, a sea turtle heads into the sea to start a voyage that may cover thousands of kilometers. Turtles navigate by sensing magnetic fields. For East-West navigation, a turtle senses slight variations in the angle at which the Earth’s magnetic field intersects the Earth’s surface, employing the feel of quantum effects to do so.
Relying upon magnetic field detection, humpback whales may migrate for thousands of kilometers in a straight line.
Grazing cow herds tend to align along geomagnetic field lines, as do dogs. Magnetic-field detection is prevalent even in species that are not migratory.