Bird Migration: The Longest Journeys in the Animal Kingdom -- Arctic Terns, Godwits, and the Science of Epic Flight
Twice a year, roughly 50 billion individual birds lift off from every continent on Earth and relocate themselves across hemispheres. They cross oceans without a compass, deserts without water, and mountain ranges without supplemental oxygen. Some fly for days without landing. Some climb higher than commercial aircraft. Some navigate by the stars, some by the Earth's magnetic field, and some by the smell of the wind. Bird migration is the largest coordinated movement of biomass on the planet, and despite centuries of study, it continues to produce discoveries that force ornithologists to revise what they thought they knew about the limits of animal endurance.
Migration is not a simple phenomenon. It is not merely flying south for winter and north for summer. It encompasses altitudinal movements in mountain ranges, longitudinal shifts along coastlines, loop migrations that follow different outbound and return routes, and irruptive movements driven by unpredictable food shortages. Some species migrate 44,000 miles annually. Others move 300 feet downhill. Both are migration, and both are shaped by the same evolutionary pressures that have been sculpting avian behavior for at least 50 million years.
Understanding migration matters far beyond academic ornithology. Migratory birds are vectors for seed dispersal, pest control, and nutrient transfer across ecosystems. They are sentinels of environmental change -- when migration timing shifts or routes collapse, it signals disruptions that inevitably affect agriculture, fisheries, and human health. The study of bird migration is, in a very real sense, the study of planetary health.
Why Birds Migrate: The Three Drivers
The fundamental question -- why do birds migrate at all, rather than staying put year-round? -- has a deceptively simple answer rooted in three interconnected pressures: food availability, breeding opportunity, and daylight.
Food Availability
The primary driver of migration is the seasonal redistribution of food resources. Temperate and Arctic regions produce extraordinary pulses of insect biomass during their long summer days, but those resources collapse to near zero in winter. A warbler that thrives on caterpillars in a Canadian boreal forest in June would starve in the same forest in January. Migration allows birds to exploit the seasonal abundance of high latitudes while avoiding the seasonal famine.
This is not limited to insect-eaters. Shorebirds follow the tidal flats and mudflats that produce invertebrate prey at predictable times. Raptors track rodent and fish populations that peak during certain seasons. Frugivores in tropical regions make altitudinal migrations to follow the ripening cycles of different plant species.
Breeding Opportunity
High-latitude regions offer critical advantages for raising young. The long daylight hours of an Arctic or sub-Arctic summer provide up to 24 hours of foraging time per day -- a decisive advantage when parent birds must feed growing chicks every few minutes. The seasonal explosion of insects in northern forests and tundra provides a concentrated, protein-rich food source ideal for nestling development. Additionally, the relatively lower density of predators and nest parasites at high latitudes reduces the risks to eggs and chicks compared to the species-dense tropics.
Daylight
Photoperiod -- the length of daylight -- is the master trigger for migration. As day length changes with the seasons, specialized photoreceptors in the bird's hypothalamus detect the shift and initiate a cascade of hormonal changes. Increasing day length in spring triggers the release of gonadotropins that stimulate reproductive development and the restless, directional behavior known as Zugunruhe -- migratory restlessness. Decreasing day length in autumn reverses the process, triggering hyperphagia (intense feeding to build fat reserves) and the southward urge. This photoperiodic trigger is so reliable that captive birds kept in constant artificial light still show Zugunruhe behavior on the same schedule as their wild counterparts, driven by an internal circannual clock [1].
The Arctic Tern: The Longest Migration on Earth
No animal on Earth travels farther in a year than the Arctic tern (Sterna paradisaea). This small seabird, weighing barely 100 grams, undertakes an annual round trip from its breeding grounds in the Arctic to its wintering grounds in the Antarctic and back -- a journey of approximately 44,000 miles (71,000 km) along a winding, figure-eight route that follows prevailing wind patterns across the Atlantic.
A landmark 2010 study by Egevang et al., published in the Proceedings of the National Academy of Sciences, used miniaturized geolocators to track Arctic terns from breeding colonies in Greenland and Iceland. The data revealed that the birds did not follow the shortest path between poles. Instead, they traced a looping S-shaped route down the Atlantic, veering toward the coast of West Africa before crossing to South America, then sweeping east to reach the Antarctic [2]. The detour added thousands of miles but allowed the terns to exploit prevailing tailwinds and productive upwelling zones for feeding along the way.
The Arctic tern's migration route means it experiences two polar summers per year -- more daylight than any other creature on the planet. It breeds in the continuous daylight of the Arctic summer, then flies to the Antarctic where it arrives in time for the austral summer. Over its typical 30-year lifespan, a single Arctic tern may fly the equivalent of 1.5 million miles -- roughly three round trips to the moon.
"The Arctic tern sees more daylight in a year than any other animal on the planet. It lives in a world of almost perpetual summer, chasing the sun from pole to pole in a journey that makes every other migration look like a commute." -- Carsten Egevang, Greenland Institute of Natural Resources (2010)
The tern accomplishes this feat with a body that weighs less than a smartphone. Its long, narrow wings are optimized for efficient, low-energy soaring, and it feeds on small fish and invertebrates snatched from the ocean surface throughout its journey. Unlike many migrants that must build massive fat reserves before departure, the Arctic tern refuels continuously along its route, turning the entire Atlantic Ocean into a chain of service stations.
The Bar-Tailed Godwit: The Longest Non-Stop Flight
If the Arctic tern holds the record for total annual distance, the bar-tailed godwit (Limosa lapponica) holds what may be the more physiologically astonishing record: the longest non-stop flight of any bird, covering more than 7,000 miles (11,000 km) without rest, food, or water.
The record was established in 2007 when a female bar-tailed godwit designated E7 was fitted with a satellite transmitter at her staging ground in Alaska. When E7 departed the Yukon-Kuskokwim Delta in September, she climbed to altitude, turned south, and flew continuously for eight days and eight nights across the open Pacific Ocean, landing on the mudflats of the Firth of Thames in New Zealand after covering approximately 7,145 miles (11,500 km) without once stopping to eat, drink, or sleep [3]. She had no island stops available across that vast oceanic crossing -- it was continuous powered flight or death by drowning.
In 2020, a male bar-tailed godwit designated 4BBRW broke E7's record, flying non-stop from Alaska to New Zealand over 12,200 km (7,580 miles) in approximately 11 days, as tracked by satellite telemetry.
The physiological preparations for this flight are extraordinary. In the weeks before departure, godwits undergo a metabolic transformation. They double their body weight by depositing massive fat reserves. Their flight muscles enlarge dramatically. Most remarkably, their internal organs -- kidneys, liver, intestines -- shrink by as much as 25 percent, effectively sacrificing digestive capacity (which is unnecessary during a flight with no feeding stops) to reduce weight and make room for additional fat storage. The bird literally restructures its body for the journey.
During the flight itself, godwits maintain an average speed of approximately 35 mph while navigating over featureless open ocean with no landmarks. How they maintain course across 7,000 miles of Pacific without visual reference points remains one of the great questions in animal navigation.
Monarch Butterflies: A Migration Comparison
Bird migration invites comparison with the most famous insect migration: the monarch butterfly (Danaus plexippus), which travels up to 3,000 miles from breeding grounds in the northern United States and Canada to overwintering sites in the oyamel fir forests of central Mexico.
The monarch's journey is remarkable for a different reason than distance. No individual monarch completes the full round trip. The northward spring migration takes three to four generations, with each generation living just a few weeks, mating, laying eggs, and dying before the next generation continues the journey. Only the autumn generation -- the so-called "super generation" -- makes the full southward journey and survives the winter, living up to eight months before mating in spring and beginning the cycle again.
While monarchs cover far less distance than Arctic terns or godwits, they do it with a brain containing roughly one million neurons (compared to a bird's hundreds of millions), a body weighing less than a gram, and wings made of chitin rather than feathered bone. Their navigation relies on a time-compensated sun compass housed in their antennae, combined with a magnetic sense whose mechanism is still debated.
V-Formation Aerodynamics: The Physics of Energy Saving
One of the most recognizable sights of migration season is a skein of geese or pelicans flying in V-formation against an autumn sky. This formation is not aesthetic coincidence -- it is a precisely calibrated aerodynamic strategy that measurably reduces the energy cost of flight.
When a bird flaps its wings, each wingtip generates a rotating vortex of air -- a downwash directly behind the wing and an upwash on the outer edge. The upwash zone provides a column of rising air that can partially support the weight of any bird positioned within it. By flying in V-formation, each trailing bird positions itself in the upwash generated by the bird ahead, gaining free lift that reduces the effort required to stay airborne.
A 2001 study by Weimerskirch et al. fitted great white pelicans with heart rate monitors and found that birds flying in formation had heart rates 11.4 to 14.5 percent lower than birds flying solo -- a direct proxy for reduced metabolic effort [4]. Subsequent research on northern bald ibises, published in Nature in 2014, used GPS loggers and accelerometers to demonstrate that ibises in V-formation synchronized their wingbeats with extraordinary precision, timing their flaps to arrive in the upwash peak of the bird ahead. The energy savings were estimated at 12 to 20 percent [5].
The lead position in a V-formation offers no drafting benefit, and studies confirm that birds rotate the lead position regularly, sharing the energetic burden. This cooperative behavior is most developed in species that migrate in stable family groups or long-term flocks, such as geese and cranes, where the social bonds that enable trust and cooperation have time to develop.
Navigation: Five Senses and a Magnetic Compass
How a bird weighing a few ounces finds its way from a Canadian marsh to a specific tree in a Costa Rican forest -- and back again the following spring -- is one of the most intensely studied questions in animal behavior. The answer involves at least five overlapping navigation systems.
The Magnetic Sense
Birds can detect the Earth's magnetic field. The mechanism appears to involve cryptochrome proteins in the retina of the eye, which undergo quantum-level changes in electron spin when exposed to magnetic fields. This process, known as the radical pair mechanism, essentially allows the bird to "see" the magnetic field as a visual overlay -- a pattern of light or shadow superimposed on its normal vision. The inclination angle of the field lines provides latitude information, while the field intensity provides a rough geographic position [6].
Experimental evidence for magnetic navigation is extensive. Birds placed in Emlen funnels (circular enclosures that record the direction a bird attempts to fly) orient correctly under natural magnetic conditions but become disoriented when the ambient field is artificially rotated or disrupted. European robins exposed to weak radiofrequency electromagnetic noise -- the kind produced by electronic devices -- lost their magnetic orientation entirely, suggesting that the quantum compass is exquisitely sensitive.
The Sun Compass
Birds use the sun's position as a directional reference, calibrated by their internal circadian clock. Because the sun moves across the sky throughout the day, a bird must compensate for the time of day to extract a consistent directional reading. Experiments have demonstrated this time-compensation mechanism by clock-shifting birds -- advancing or delaying their internal clocks by several hours and then releasing them. Clock-shifted birds consistently orient in the wrong direction by an angle predictable from the clock offset, confirming that the sun compass is time-dependent.
Star Navigation
Nocturnal migrants -- the majority of songbird species -- navigate by the stars. Research by Stephen Emlen in the 1960s and 1970s demonstrated that indigo buntings learn the pattern of stellar rotation around the celestial north pole during their first summer and use it as a directional reference for the rest of their lives. Buntings raised under artificial planetarium skies with a shifted rotational axis oriented their migratory direction toward the artificial "north," confirming that the reference is learned, not innate [7].
Olfactory Maps
Pigeons and some seabirds build atmospheric odor maps that allow them to navigate over featureless terrain or ocean. Research on Cory's shearwaters has shown that birds with temporarily blocked olfactory senses are unable to find their way home from unfamiliar locations, while control birds with intact smell navigate accurately. The odor map is thought to be constructed from gradients of atmospheric trace chemicals carried by prevailing winds.
Landmark Recognition
Experienced migrants supplement their compass senses with visual recognition of geographic features -- coastlines, river valleys, mountain ranges, and even urban areas. This is why experienced adult birds navigate more accurately than juveniles on their first migration. The cognitive map built over successive journeys allows older birds to correct for wind drift and make route adjustments that young birds cannot.
The Four Great Flyways
Migratory birds do not scatter randomly across the globe. They funnel along broad corridors known as flyways -- routes shaped by geography, wind patterns, food availability, and the locations of critical stopover sites. Four major flyways span the Americas.
The Atlantic Flyway runs along the eastern seaboard from Arctic Canada to the Caribbean and South America. It channels billions of shorebirds, songbirds, and raptors through bottleneck sites like Cape May, New Jersey, and the Delmarva Peninsula. The Delaware Bay, where horseshoe crabs spawn each May, is a critical refueling stop for red knots and other shorebirds that time their passage to coincide with the egg-laying.
The Mississippi Flyway follows the Mississippi River valley from the boreal forests of central Canada to the Gulf of Mexico and beyond. It is the most heavily used flyway in North America, carrying an estimated 40 percent of all migratory waterfowl on the continent. The river valley provides a continuous corridor of wetland habitat that stretches nearly 3,000 miles without major geographic barriers.
The Central Flyway spans the Great Plains from the Canadian prairies to the Texas coast and into Mexico. It is critical for grassland-nesting species and carries enormous numbers of sandhill cranes, which stage each spring along the Platte River in Nebraska in congregations numbering over 500,000 individuals -- one of the great wildlife spectacles on Earth.
The Pacific Flyway follows the western coast from Alaska to Patagonia. It carries vast numbers of shorebirds, including the bar-tailed godwits that make their non-stop transpacific crossings, as well as the gray whales and humpback whales that share many of the same coastal staging areas.
Record-Breaking Migrants: A Comparison
The following table compares the most extraordinary migratory feats recorded in the animal kingdom.
| Species | Record | Distance | Notable Detail |
|---|---|---|---|
| Arctic tern | Longest annual migration | 44,000 miles (71,000 km) round trip | Experiences two polar summers; lifetime distance equals 3 trips to the moon |
| Bar-tailed godwit (4BBRW) | Longest non-stop flight | 7,580 miles (12,200 km) | 11 days without food, water, or rest over open Pacific Ocean |
| Bar-headed goose | Highest-altitude migration | 29,000 feet (8,800 m) | Crosses the Himalayas in a single overnight flight |
| Sooty shearwater | Longest tracked seabird migration | 40,000 miles (64,000 km) annual loop | Figure-eight route across the entire Pacific Ocean |
| Ruby-throated hummingbird | Longest non-stop flight relative to body size | 500 miles across Gulf of Mexico | Weighs 3 grams; burns fat reserves equivalent to crossing 18 hours non-stop |
| Monarch butterfly | Longest insect migration | 3,000 miles (4,800 km) one way | Multi-generational northward; single generation returns south |
| Globe skimmer dragonfly | Longest insect migration (recently measured) | 11,000 miles (18,000 km) multi-generational | Crosses the Indian Ocean; possibly the longest insect migration on Earth |
Altitude Records: Bar-Headed Geese Over the Himalayas
The bar-headed goose (Anser indicus) performs what may be the most physically demanding migration on the planet: a direct crossing of the Himalayan mountain range at altitudes exceeding 29,000 feet (8,800 m) -- the cruising altitude of commercial jetliners and the height of Mount Everest itself.
At 29,000 feet, the air contains only about one-third the oxygen available at sea level. Temperatures drop to minus 50 degrees Celsius. Wind speeds can exceed 200 mph in the jet stream. Yet bar-headed geese cross this barrier routinely, often completing the Himalayan transit in a single overnight flight lasting 7 to 8 hours.
The adaptations that enable this feat are multiple. Bar-headed geese possess a variant of hemoglobin with a higher oxygen affinity than that of lowland geese, allowing their blood to bind oxygen more efficiently in thin air. Their capillary networks in the flight muscles are denser than those of other geese, delivering oxygen more effectively to working tissue. Their lungs, like all bird lungs, use a unidirectional flow-through system that extracts oxygen far more efficiently than the tidal ventilation of mammalian lungs -- an adaptation that becomes critical when every molecule of available oxygen matters.
"Bar-headed geese do not acclimatize to altitude. They simply fly through it. They ascend from near sea level to the height of Everest in hours, process one-third the normal oxygen, and arrive on the other side. No mammal could do what these birds do routinely." -- Lucy Hawkes, University of Exeter (2013)
Research by Hawkes et al. using GPS transmitters and pressure altimeters revealed that bar-headed geese adopt a "roller coaster" flight profile, closely following the terrain rather than climbing to a single high altitude and maintaining it. This strategy minimizes time at maximum altitude while exploiting the denser air in valleys between peaks [8].
Dangers Facing Migratory Birds
Migration is inherently dangerous -- it is estimated that up to 50 percent of migratory songbirds do not survive their first round trip. But human activity has added an array of new hazards that dwarf the natural risks.
Building Collisions
Glass-covered buildings are invisible death traps for migratory birds. An estimated 600 million birds die each year in the United States alone from collisions with buildings, according to a 2014 study published in The Condor [9]. Birds see reflected sky or vegetation in the glass and fly into it at full speed. Low-rise residential buildings account for the majority of deaths by sheer number, but high-rise buildings in urban cores -- particularly those illuminated at night -- can kill hundreds of birds in a single night during peak migration.
Light Pollution
Artificial light at night disorients nocturnal migrants, drawing them toward illuminated structures where they circle in confusion until they collide with buildings, exhaust themselves, or fall prey to predators. The September 11 Memorial Tribute in Light in New York City, which projects two intense columns of light into the sky each anniversary, has been documented trapping thousands of migratory birds in its beams. The installation is now monitored by the NYC Audubon Society, which requests that the lights be temporarily extinguished when bird concentrations exceed dangerous thresholds.
Cities across North America have adopted Lights Out programs, encouraging building managers to extinguish or dim unnecessary lighting during peak migration periods. Toronto, Chicago, Houston, and Philadelphia have all implemented voluntary or mandatory light reduction programs that have demonstrably reduced collision mortality.
Wind Turbines
Wind energy facilities kill an estimated 140,000 to 500,000 birds annually in the United States, according to a 2013 study in Biological Conservation. Raptors and other soaring birds are disproportionately affected because they use the same ridgelines and thermal corridors that wind developers favor. The Altamont Pass Wind Resource Area in California became infamous for killing an estimated 4,700 birds annually, including golden eagles, red-tailed hawks, and burrowing owls, before older turbine designs were replaced with larger, slower-spinning models that reduced raptor mortality.
Habitat Loss at Stopover Sites
Migratory birds depend on a chain of stopover habitats along their routes -- wetlands, forests, and coastal areas where they refuel for the next leg of their journey. The loss of any critical link in that chain can be catastrophic. The destruction of Yellow Sea tidal flats in East Asia -- where land reclamation for development has eliminated over 65 percent of intertidal habitat since the 1950s -- has been linked to severe population declines in multiple shorebird species that depend on those flats during migration.
Radar Tracking and Satellite Telemetry: The Technology Revolution
The study of bird migration has been transformed by technology. Traditional methods -- banding, visual observation, and recovery of dead birds -- provided snapshots of where birds had been but could not reveal the routes, altitudes, speeds, and behaviors of birds in active flight. Modern technology has changed that entirely.
Weather surveillance radar (WSR-88D) networks, originally designed to track precipitation, have proven remarkably effective at detecting and quantifying bird migration. The BirdCast project, a collaboration between the Cornell Lab of Ornithology, Colorado State University, and the University of Massachusetts, uses machine learning algorithms to analyze radar data in real time, producing nightly migration forecasts that predict the intensity, altitude, and direction of bird movements across the entire continental United States. On peak autumn nights, radar detects the movement of hundreds of millions of birds simultaneously.
Satellite telemetry using GPS and Argos transmitters has revolutionized individual tracking. Devices that once weighed 30 grams -- too heavy for all but the largest birds -- now weigh as little as 0.3 grams, enabling researchers to track songbirds, hummingbirds, and even large insects. The Motus Wildlife Tracking System, a collaborative network of over 1,500 automated radio telemetry stations across the Americas, provides continent-scale tracking of small-bodied migrants equipped with nanotags weighing as little as 0.12 grams.
Geolocators, which record ambient light levels to calculate latitude and longitude, were the technology that revealed the Arctic tern's full migration route for the first time in 2010. While less precise than GPS (accurate to roughly 100-200 km rather than meters), their minuscule size and weight make them suitable for species too small for satellite transmitters.
These technologies have produced discoveries that were impossible just two decades ago. Satellite tracking revealed that the great snipe -- a stocky shorebird -- makes non-stop flights of over 4,000 miles at sustained speeds of 60 mph, the fastest sustained migration speed ever recorded. Radar analysis revealed that the magnitude of nocturnal songbird migration over the Gulf of Mexico dwarfed previous estimates by an order of magnitude. Geolocators showed that common swifts can remain airborne continuously for up to 10 months without landing, sleeping on the wing.
Climate Change: Shifting the Calendar
Climate change is altering the timing of migration in measurable and concerning ways. Spring temperatures in the Northern Hemisphere have advanced by approximately 2 to 4 days per decade over the past 50 years, and many migratory species have responded by arriving at their breeding grounds earlier. A comprehensive analysis of European bird migration data found that short-distance migrants have advanced their spring arrival by an average of 6 to 9 days over the past three decades, while long-distance migrants from sub-Saharan Africa have shifted by only 2 to 3 days [10].
This asymmetry creates a dangerous mismatch. The insects and plants that migratory birds depend on for food have also shifted their phenology in response to local temperature changes -- but these shifts do not always match the shifts in bird arrival. When a migratory bird arrives at its breeding ground after the peak emergence of caterpillars that it needs to feed its chicks, reproductive success drops sharply. This phenomenon, known as phenological mismatch, has been documented in pied flycatchers, great tits, and numerous other species.
Climate change is also altering migration routes. As temperatures warm, the ranges of many species are shifting northward, and some populations that were previously migratory are becoming sedentary. European blackcaps that once wintered in West Africa are increasingly overwintering in Britain, where milder winters and supplemental feeding at garden bird feeders provide sufficient resources to survive. In North America, the number of bird species observed on Christmas Bird Counts at northern latitudes has increased steadily, suggesting a broad-scale northward shift in wintering ranges.
The long-term consequences of these shifts are uncertain. Species that can adjust their migration timing rapidly may benefit from expanded breeding ranges and reduced migration costs. Species that cannot -- particularly long-distance migrants whose departure timing is controlled by photoperiod rather than local temperature -- face increasing mismatches between their arrival and the availability of food, potentially driving population declines that are already visible in monitoring data.
The Ancient and Ongoing Journey
Bird migration has been occurring for tens of millions of years. Fossil evidence and phylogenetic analysis suggest that migratory behavior evolved independently in multiple avian lineages, likely in response to the climatic oscillations of the Pleistocene that repeatedly expanded and contracted suitable habitat across the Northern Hemisphere. The flyways that birds follow today are not fixed -- they are living corridors that have shifted, merged, and diverged as ice sheets advanced and retreated, continents drifted, and ecosystems reorganized.
What has not changed is the fundamental calculus that drives migration: the trade-off between the risks of the journey and the rewards of seasonal abundance at its end. A bar-tailed godwit that flies 7,000 miles non-stop across the Pacific does so because the feeding grounds of New Zealand's tidal flats are worth the gamble. An Arctic tern that chases the sun from pole to pole does so because perpetual summer means perpetual food. A bar-headed goose that flies over the Himalayas at 29,000 feet does so because the wetlands of the Indian subcontinent are on the other side.
These journeys are not instinct in the simplistic sense that word often implies. They are the product of millions of years of natural selection acting on physiology, neurology, endocrinology, and behavior simultaneously. Every migratory bird is a living integration of aerodynamic engineering, metabolic efficiency, sensory precision, and navigational computing that no human technology has yet fully replicated or completely understood.
The study of migration continues to accelerate. Satellite tracking, genomic analysis, radar surveillance, and machine learning are revealing new dimensions of migratory behavior faster than researchers can publish their findings. But the central wonder remains the same one that captivated Aristotle when he watched cranes departing Greece each autumn: how does a creature weighing less than a handful of coins find its way across a planet?
The answer, it turns out, is that it uses everything -- magnetism, starlight, sunlight, smell, memory, physics, and a courage so embedded in its genome that it cannot be called anything other than what it is. It is the longest journey in the animal kingdom, and it happens over our heads every year, largely unseen, entirely extraordinary.
References
Gwinner, E. (1996). Circannual clocks in avian reproduction and migration. Ibis, 138(1), 47-63.
Egevang, C., Stenhouse, I.J., Phillips, R.A., Petersen, A., Fox, J.W., & Silk, J.R.D. (2010). Tracking of Arctic terns Sterna paradisaea reveals longest animal migration. Proceedings of the National Academy of Sciences, 107(5), 2078-2081.
Gill, R.E., Tibbitts, T.L., Douglas, D.C., Handel, C.M., Mulcahy, D.M., Gottschalck, J.C., Warnock, N., McCaffery, B.J., Battley, P.F., & Piersma, T. (2009). Extreme endurance flights by landbirds crossing the Pacific Ocean: ecological corridor rather than barrier? Proceedings of the Royal Society B, 276(1656), 447-457.
Weimerskirch, H., Martin, J., Clerquin, Y., Alexandre, P., & Jiraskova, S. (2001). Energy saving in flight formation. Nature, 413(6857), 697-698.
Portugal, S.J., Hubel, T.Y., Fritz, J., Heese, S., Trobe, D., Voelkl, B., Hailes, S., Wilson, A.M., & Usherwood, J.R. (2014). Upwash exploitation and downwash avoidance by flap phasing of free-flying northern bald ibises. Nature, 505(7483), 399-402.
Mouritsen, H. (2018). Long-distance navigation and magnetoreception in migratory animals. Nature, 558(7708), 50-59.
Emlen, S.T. (1975). The stellar-orientation system of a migratory bird. Scientific American, 233(2), 102-111.
Hawkes, L.A., Balachandran, S., Batbayar, N., Butler, P.J., Frappell, P.B., Milsom, W.K., Tseveenmyadag, N., Newman, S.H., Scott, G.R., Sathiyaselvam, P., Takekawa, J.Y., Wikelski, M., & Bishop, C.M. (2013). The paradox of extreme high-altitude migration in bar-headed geese Anser indicus. Proceedings of the Royal Society B, 280(1750), 20122114.
Loss, S.R., Will, T., Loss, S.S., & Marra, P.P. (2014). Bird-building collisions in the United States: Estimates of annual mortality and species vulnerability. The Condor, 116(1), 8-23.
Saino, N., Ambrosini, R., Rubolini, D., von Hardenberg, J., Provenzale, A., Huppop, K., Huppop, O., Lehikoinen, A., Lehikoinen, E., Rainio, K., Romano, M., & Sokolov, L. (2011). Climate warming, ecological mismatch at arrival and population decline in migratory birds. Proceedings of the Royal Society B, 278(1707), 835-842.
Frequently Asked Questions
How far does the Arctic tern migrate each year?
The Arctic tern holds the record for the longest migration of any animal on Earth, traveling approximately 44,000 miles (71,000 km) annually on a round trip from its Arctic breeding grounds to the Antarctic and back. Over its 30-year lifespan, a single Arctic tern may fly the equivalent of three round trips to the moon -- roughly 1.5 million miles. By chasing the polar summers at both ends of the planet, the Arctic tern experiences more daylight than any other creature on Earth.
How do birds navigate during migration?
Birds use multiple overlapping navigation systems to find their way during migration. These include a magnetic sense that detects the Earth's magnetic field through cryptochrome proteins in the eye, a sun compass calibrated by an internal circadian clock, star navigation using the rotation of constellations around the celestial pole, olfactory maps built from atmospheric odor gradients, and visual landmark recognition. Young birds on their first migration often rely on innate compass directions, while experienced adults build detailed cognitive maps of their routes over successive journeys.
How does flying in V-formation save energy for birds?
When a bird flaps its wings, it creates a rotating vortex of air behind each wingtip -- an upwash on the outer edge that provides free lift to any bird positioned there. By flying in V-formation, trailing birds position themselves in the upwash zone of the bird ahead, reducing the aerodynamic effort required to stay aloft. Research has shown this saves between 12 and 20 percent of the energy a bird would expend flying alone. Birds rotate the lead position because the front bird receives no drafting benefit and tires faster than those behind it.