Wings on the wind
How do migrating birds know exactly when, and where, to go?
Posted on homepage: 17 October 2012 (GMT+10)
Imagine if, when you were very young and were utterly dependent on your parents for protection and food, they had abandoned you suddenly to go overseas? Without any written instructions as to where they’d gone, would you have been able to follow their path once you’d grown strong enough to try?
Sound impossible? Not for the Bristle-thighed Curlew! When the chicks are just five weeks old, the parents depart, heading for the tropics. 1 Left behind in the marshes of the Alaska Peninsula, the chicks gorge themselves on berries and insects. As their little bodies become stronger, accumulating the all-important reserves of fat—fuel for the long journey ahead—they frequently take to the air for short flights, as if in premigratory practice.
Then one day, the birds launch themselves into the sky, and, finding the right winds, head off on the long nonstop flight south to their ancestral wintering grounds. As with most migratory bird species, the curlew novices are on their own, without a guide. Their parents and experienced elders have departed weeks earlier. Yet most 2 of these first-year curlew pilots will unerringly navigate the vast Pacific Ocean, descending with pinpoint accuracy onto the mudflats and sandy beaches of islands in Fiji, Tonga and French Polynesia—their new home. 3
The chicks of another famous migratory species, the Short-tailed Shearwater (‘muttonbird’), 4, 5 must also navigate on their first flight without the assistance of experienced guides. 6 Breeding in burrows on islands off southeastern Australia, the parents suddenly desert the chicks at the end of summer. The parents head north, riding the prevailing winds that will take them around the western Pacific Ocean past Japan and Siberia, east around Alaska, and south down the western United States, before they return across the Pacific for the next start of the Australian summer breeding season.
Without parents to bring them food, the abandoned shearwater chicks live on accumulated fat for about two weeks. Becoming restless, they leave their burrows to test their wings in the night breeze. Soon they find a suitable takeoff point on a cliff or overhang, and plunge into their new element, somehow charting their way over vast unfamiliar oceans to the other side of the world, months later finding their way back to the very same island to breed. Birdwatchers along the eastern Australian coast have observed as many as 60,000 shearwaters per hour flying past, arriving back at their burrows within the same eleven-day period each year.
The ability of both juvenile curlews and shearwaters to navigate, untaught, to the opposite hemisphere is astounding, but of the two, the curlew’s is probably the more remarkable. While the Short-tailed Shearwater is renowned for the length of its great journey (over 13,000 km (8,000 miles) from Australia to Alaska), the Bristle-thighed Curlew’s flight of more than 8,000 km (5,000 miles) across the Pacific is nonstop. Unlike seabirds like terns or shearwaters, which can rest and feed along the way, the curlews will drown if they land on the ocean.
But the curlew’s incredible migration is exceeded by the Bar-tailed Godwit. In midsummer, godwits nesting in western Alaska leave their breeding grounds and congregate by the tens of thousands along the Alaska Peninsula, where they feed on clams and other goodies from the intertidal mudflats. They gorge themselves until the fat builds up into thick rolls under their skin—up to 55% of their total weight.
Then they stop eating, and undergo an incredible internal change. Their intestines, kidneys and liver shrivel up, shrinking to a fraction of their usual size (scientists suspect this happens to many long-distance migrants). Laden with fuel, and with lightened innards, the godwits take off by the thousands, flying south at around 72 km per hour (45 mph). Many will not stop again until they reach New Zealand, a journey of 11,000 km (6,800 miles), lasting four or five days—believed to be the longest nonstop bird migration in the world.
‘South for the winter’?
Just why do birds embark on these incredible journeys? The popular Northern hemisphere notion that ‘birds fly south for the winter’ is somewhat misleading.
Birds do not leave an area merely because the weather is due to turn cold. Studies show that migration is fundamentally about food supply, not temperature, as birds that can continue to find enough to eat during the winter rarely migrate—e.g. many ravens, which eat almost anything, have been known to survive in areas where the temperature drops to –57°C
In contrast, almost all the bird species that do migrate depend upon weather-sensitive food supplies. Insect-eating songbirds would have a hard time trying to find bugs once the winter snows arrived; similarly, wading birds, once their marshes became sealed by ice. Birds that eat seeds are less likely to migrate than insectivorous birds, and tend not to travel as far. Among the insect-eaters, too, there are differences—birds that eat flying insects must migrate to significantly warmer or even tropical areas, in contrast to birds that eat terrestrial insects.
So a determining factor in migration is the deliberate moving towards something beneficial rather than a moving away from something unpleasant. It’s not just a simple, north-to-south-and-back-again affair either. As the Short-tailed Shearwaters show, the oceans of the world are criss-crossed by masses of migrating birds, few of which are traveling to avoid bad weather. Often they are traveling from isolated breeding islands to rich feeding grounds close to the opposite pole. 7 The Arctic Tern’s quest for food takes it over 35,000 km (22,000 miles) in a single year—the world’s longest migration route. Nesting at high northern latitudes, it annually travels south to fish through the Antarctic summer, enjoying a greater percentage of daylight in its life (thus more hours in which to hunt) than any other creature on Earth.
Over land, migration can have a strong east-to-west component, e.g. in the USA, Red-head Ducks may migrate from Utah to the Atlantic for the winter, while Harlequin Ducks that nest in the Rocky Mountains migrate west to the Pacific. Within the tropics, hummingbirds, parrots and toucans [pictured, left] undertake great migrations which often coincide with the mass blossoming of nectar-laden flowers or the ripening of fruit or grass seeds. In the mountains of Central and South America, it is estimated that a fifth of the tropical bird species seasonally migrate between highlands (where nectar and fruit are abundant only part of the year) and warmer, wetter lowlands.
Of course, the fluctuations in food supply (seeds, nectar, insects), which apparently spur bird migration, are themselves tied to the seasonal variation in the Earth’s climate. This seasonal variation occurs because our planet’s axis is tilted off the vertical by an average of 23.5°, so as the Earth orbits the sun, first the Northern hemisphere then the Southern hemisphere are inclined toward the sun, producing the seasons. 8
Despite being closely linked to food supply, not temperature, it appears that regular bird migration journeys are not usually driven by hunger. In fact, for most migrating species, a bird is at its fattest just before beginning its migration—notably the Bar-tailed Godwit.
Traveling by their own power over several thousand kilometres nonstop, the godwits and other long-distance migrants demonstrate a feat of strength and endurance that far outranks other animals or man. The key to this is the reservoirs of energy-providing fat built up during the premigratory feeding frenzy. The fat is secreted into special cavities between tissues and organs. It therefore does not affect the weight or bulk of muscles that must remain in top-class condition for flying.
And this is no ordinary fat. Ordinary fat contains much water, but fat stored by migratory birds is much more highly concentrated, containing little water and thus is much lighter. But isn’t less water a problem for a long-distance flyer, unable to stop for a drink? No—because as the fat ‘burns’, combining with oxygen, enough water is produced to enable the birds to fly for long periods without drinking.
Incredibly, it appears that these long-distance migrants store exactly the right amount of fat required for the journey. The Golden Plover, for example, puts on an extra 70 g (2.4 oz) of fat, 50% of its normal bodyweight, which is precisely the right amount it needs to travel from Alaska to Hawaii, a journey of 4,500 km (2,800 miles), taking around 88 hours at about 51 km per hour (32 mph). 9 Also, it’s as if the amount of fat stored has somehow been ‘calculated’ to allow for the energy-saving boost in efficiency on these long-distance flights, compared to normal flight—an efficiency arising from ‘flying in formation’.
Birds of a feather
A Golden Plover converts 0.6% of its bodyweight into motion and heat per flight hour. (By comparison, a helicopter and a jet plane need, in relation to their weight, seven times and 20 times more fuel respectively, than a Golden Plover.)9, 10 This means that with 70 g (2.4 oz) of accumulated fat at the start of its flight, a single Golden Plover would crash into the sea 800 km short of its destination, Hawaii. But, in reality, the tropical haven is attained safely because when the Golden Plover flies over ocean, it flies in flocks in the classical ‘V-formation’, in which, on average, each bird saves 23% of the energy that would be used when flying alone. (This is not the case for the bird at the apex, 11 but birds take turns at that position, and thus ‘share the load’—something Christians are also exhorted to do (Galatians 6:2).)
Not all migrating birds fly in the energy-saving ‘V-formation’, but many birds do migrate in (often huge) flocks. In one night, radar showed an estimated 12 million songbirds passing Cape Cod, USA, on their way south. In November 1995, an estimated 50 to 80 million ducks and geese pushing south overwhelmed the air traffic control radar at the Kansas City and Omaha airports.
Time to fly
Somehow birds know when to migrate, as well as where. 12 Scientific knowledge about bird migration, particularly the ocean-goers, is still scanty, but a picture of the factors that might trigger migration is emerging. It is now believed that before a bird migrates, there must be at least two elements present: a genetic predisposition 13 and one or more environmental triggers.
Researchers have identified in songbirds that changing day length is a major environmental factor, but it is now being realized that there is a whole suite of interacting factors, such as barometric pressure, temperature, windspeed and direction. The heaviest hawk flights, for example, occur after the passage of a cold front, with lowering temperatures, rising barometric pressure, and associated brisk updrafts. In the case of the aforementioned ducks and geese which closed the Midwestern US airports, they were observed the previous day to be pouring south under sunny skies, i.e. the day before an especially strong blizzard blasted out of the Canadian prairies, pushing the birds along like a giant wave. It has been suggested that, perhaps by sensing changes in atmospheric pressure, birds can perceive the approach of major weather systems.
A built-in compass
The ability of migratory birds to fly to their destination with such precision requires two abilities: orientation (knowing direction) and navigation (knowing when to change direction). The first requires some kind of compass; the second needs a map. One without the other is useless—it seems migratory birds have both. The mechanism of birds’ innate direction-finding capacity has puzzled scientists for years. At various times it has been mooted that birds navigate by the sun, the stars, and geographical landmarks. All of these have been shown to be true but these abilities all appear to be learned by experience—e.g. pigeons raised out of sight of the sun and exercised only on overcast days cannot navigate by the sun, but can still easily find their way. Conversely, pigeons that had learned to navigate by ‘solar compass’ have a harder time navigating on cloudy days.
So while birds apparently can learn to use a whole range of environmental cues for navigation, there is considerable evidence that birds primarily use some kind of built-in ‘magnetic compass’. It has even been suggested they carry some kind of built-in ‘magnetic map’ of the Earth. Certainly, they are sensitive to the slightest differences in intensity of the Earth’s magnetic field. How can this be? Following the discovery of magnetite crystals in magnetically sensitive bacteria in the 1970s, magnetite has also been found in the nasal cavities of several species of migratory birds (as well as honeybees and other organisms with a ‘geomagnetic’ sense). But new evidence shows that birds do not simply fly on a constant magnetic compass course but continually change their heading so as to fly the most efficient route. (See Tripping the flight fantastic.)
Researchers now acknowledge that there is no one simple unified theory of how birds can navigate so precisely. They appear to be using a whole suite of magnetic, solar, stellar, atmospheric and geographical cues.
In the beginning … ?
How did this fantastic capacity to navigate across the globe come into being? According to the theory of evolution, birds are the result of millions of years of chance processes, mutations and natural selection. Evolutionist ornithologists suggest that migratory paths began to ‘evolve’ as birds pushed ‘farther and farther north each year as ice-age glaciers retreat[ed], returning in winter to a traditional non-breeding ground that lies further away with each generation.’ 14 Though that might sound plausible enough, it does not account for how a godwit first encountered far-flung islands such as New Zealand. And how did a Bristle-thighed Curlew first find tiny Rangiroa Atoll in the vastness of the Pacific? Nor does it explain how such migratory paths become ‘imprinted’ in the genes so that chicks can follow the ancestral migratory routes without guidance from experienced birds. The smallest migratory hummingbird, with a brain scarcely larger than a seed of corn, can navigate a flawless course over immense distances. Yet this marvel of migration is supposed to have come about by an undesigned process! 15
The creationist must also think carefully about exactly how such fantastic migratory pathways originated. Caged nightingales have been observed fluttering on the north wall of their cages in spring, and south in autumn. This urge to fly in a particular direction seems to be inherited.13 This is why migratory chicks are able to follow ancestral routes. It is easy to postulate that migratory instincts and capacities were all preprogrammed into the original created kinds. But the Earth’s geography has been massively changed by the global Flood. How could directional information useful in the pre-Flood world still be relevant afterwards?
It is possible that God programmed the original kinds with the instinct to migrate, but without a rigidly fixed ‘mental map’. In some amazing way, the programming involved the capacity to adapt to changes in topography (and presumably food supplies) in an inheritable fashion.
A possible clue to a mechanism for such adaptation is provided by blackcaps, which normally migrate from Norway and western Europe to the Mediterranean and Africa. However, since the 1950s, British birdwatchers have noticed more and more blackcaps coming to England during the winter rather than to Spain. Researchers took 40 of these birds to breed them in captivity in Germany, along with a separate group from the normal blackcap migratory population. When the offspring were monitored in special laboratory facilities, those of English birds oriented on a compass heading of 273° (i.e. towards London) while the chicks of German-caught blackcaps tried to fly on a traditional heading of 227°, i.e. towards Spain.
It is thus likely that in centuries past, a few migrant blackcaps always strayed to Britain, the victims of a mutation in their genetic code controlling orientation. Natural selection had formerly weeded these out, but in recent decades, winters in Britain have been warmer, and there has been a huge increase in winter food, e.g. backyard bird feeders.
Perhaps many of today’s migratory routes came about in such a way, as the ‘correct’ routes were selected for from among the variation built-in at Creation for this purpose. Bird migration is such a bewilderingly complex phenomenon, however, that at present, we can do little more than speculate about the details of how these incredible post-Flood migratory routes arose. Meanwhile, we would be wise to acknowledge what God’s Word tells us in relation to migration and feeding of birds.
Ultimately, we read that it is the Lord who provides food for the raven when its young cry out for lack of food (Job 38:41). Indeed, all the animals and birds look to God to give them their food at the proper time (Psalm 104:21, 24, 27–28; 136:25; 145:15–16; 147:9). In light of these verses, it is interesting to note the surprise of ornithologists who observed (across a range of species) that young migrant birds, encountering the tropics for the first time, ‘showed an almost uncanny ability to find their species-specific habitat with no discernible fumbling around.’ 16 Evolutionary ornithologists might also do well to ponder the words of rebuke that the Lord spoke to Job: ‘Does the hawk take flight by your wisdom and spread his wings towards the south?’ (Job 39:26).
But perhaps the most telling verse comes from Jeremiah 8:7, which says, ‘Even the stork in the sky knows her appointed seasons, and the dove, the swift and the thrush observe the time of their migration’, and concludes: ‘But my people do not know the requirements of the Lord.’
See you next year?
The practice of ‘banding’ birds’ ankles1 may have started almost accidentally, when Henry IV of France lost one of his trained Peregrine Falcons around 1595, and the marked bird showed up the following day on the Mediterranean island of Malta. In 1710 a grey heron carrying a leg ring from Turkey was caught in Germany by a falconer’s bird. And in the early 1800s, a Pennsylvanian resident tied a light silver thread to the leg of nesting phoebes, subsequently confirming his theory that the same individuals returned year after year.
Banding has now grown to the point where, in North America alone, more than 56 million birds have been banded in the last 100 years—three million of which were recovered.
Banding has shown not only that migratory birds often return to precisely the same tree to nest each year, but also that they show similar loyalty to their wintering site on the other side of the globe.2
- Weidensaul, S., Living on the wind: Across the hemisphere with migratory birds, North Point Press, New York, pp. 38–43, 1999.
- The homing instinct of pigeons fascinated Charles Darwin, who wondered if pigeons hauled in crates far from home somehow memorized the twists and turns in the road as they felt them. (It wasn’t until many years later that ‘retracement navigation theory’, as it was known, was finally disproved.)
Tripping the flight fantastic
Radar-tracking satellite-based telemetry allows the flight paths of individual birds tagged with tiny transmitters to be recorded continuously. Using such technology to study various migrating Arctic shorebirds has revealed an incredible fact. When migrating, these birds fly along the Earth’s great circle routes (orthodromes) rather than on a constant magnetic compass course (loxodrome), which is easier to navigate but results in longer flight distances.1,2
The great circle route conserves energy because it is the shortest distance to the final destination. But it is navigationally demanding because birds migrating along these orthodromes must continuously change their compass course because their route intersects successive longitudes.
How do they do it? Mathematicians have calculated that if one uses a time-compensated sun compass without resetting one’s internal clock while traveling across successive longitudes (i.e. moving into different time zones), the resulting route would be an orthodrome, i.e. the shortest flying distance. This navigational trick is especially beneficial to the Arctic shorebirds, because, the closer one is to the polar regions, the better this short-cut strategy works. The birds also know how to compensate for crosswinds, automatically changing their directional heading to account for any sideways deviation. There are still many more mysteries to explore, as the Arctic shorebirds do not return in spring along the same routes used in autumn—an observation the evolutionary researchers say ‘testifies to the complexity of the global orientation performance of migrating birds.’
- Wehner, R., Bird navigation—computing orthodromes, Science 291(5502):264–265, 2001.
- Alerstam, T., Gudmundsson, G., Green, M., Hedenström, A., Migration along orthodromic sun compass routes by Arctic birds, Science 291(5502):300–303, 2001.
Day or night?
Not all birds fly to their destination non-stop.1 Hawks migrate only during the daytime, gliding vast distances from one thermal to the next.
But much bird migration happens at night. And not just nocturnal species like owls, but hundreds of otherwise day-active species—sandpipers, swans, songbirds and wading birds.
For many years, man’s only clue to the extent of nighttime migration was by ‘moon-watching’—i.e. counting the silhouettes of birds passing in front of the moon over a given period.2
It was not until the 1950s that it was realized that radar could detect flocks of migrant birds. They showed up as diffuse green blobs (‘ghosts’) crossing the monitor—the same ‘radar angels’ that had puzzled, worried and even awed the military during World War II.
- Weidensaul, S., Living on the wind: Across the hemisphere with migratory birds, North Point Press, New York, pp. 16,30,35,36, 1999.
- A full moon covers about 1/347.45 of the visible sky. Therefore, five moon-silhouetted birds in 10 minutes equates to 30 per hour; multiplied by 347.45 equals about 10,420 birds per hour passing overhead in a band about 2.4 km (1½ miles) wide.
References and notes
- Weidensaul, S., Living on the wind: Across the hemisphere with migratory birds, North Point Press, New York, p. 13, 1999. Unless otherwise indicated, information presented here is sourced from this reference. Return to text.
- Hazards abound on the long journey, including storms, predators and exhaustion. Nowadays the list includes radio towers and cross-country powerlines—thousands of dead birds can be found littering the ground near these in the morning—as well as lighthouses and brightly illuminated buildings. Return to text.
- Captain Cook encountered the Bristle-thighed Curlew on Tahiti in 1769. For a hundred years afterwards, scientists assumed it was a permanent resident there, unaware of its annual sojourn in Alaska. Return to text.
- So named because they were an important source of meat (‘mutton’) for the early British settlers in Australia. Return to text.
- Shearwaters are members of a ‘family’ of great ocean-traveling birds popularly known as ‘petrels’, which also includes the many species of albatross. (The term ‘petrel’ is a diminutive meaning ‘Little Peter’—so named because the pattering of their legs and webbed feet across the surface of the water (during take-off, presumably) seemed to be an echo of the Apostle Peter’s experience of walking on water, recorded in Matthew 14:28–30.) Return to text.
- Prince, J.H., Unusual habits of Australian animals, Reed Pty Ltd, Sydney, p. 37, 1980. Return to text.
- Warm equatorial waters tend to have less food than the cooler oxygen-rich waters at high latitudes. Return to text.
- Pronounced seasonal variation has been in place at least since the time of the Flood of Noah’s day (Genesis 8:22). Seasons have existed since the Creation Week (Day 4—see Genesis 1:14), though seasonal extremes were possibly milder pre-Flood. Return to text.
- Gitt, W. and Vanheiden, K.-H., If animals could talk, Christliche Literatur-Verbreitung e.V., Bielefeld, Germany, pp. 107–115, 1994. Return to text.
- The Blackpoll Warbler, on its 5,600 km (3,500 miles) migration, will have flapped its wings about three million times. If burning standard motorcar fuel, its fuel economy would be 0.33 ml per 100 km (720,000 miles per US gal.). Return to text.
- Flying becomes progressively easier the further back a bird flies in the ‘V-formation’. Return to text.
- Not all bird migrations are annual. The Snowy Owl normally lives near the Arctic Ocean, but every decade or so it makes winter incursions as far south as Texas. This was once thought to coincide with population crashes of its favoured prey, the lemming, but recent research has cast doubt on this linkage as too simplistic. Return to text.
- Evidence for this has come from studies of various species (e.g. juncos, blackcaps) where migratory and non-migratory ‘races’ of the same subspecies occur. In blackcaps, researchers were able to selectively breed out the migratory urge within just three generations. Return to text.
- Ref. 1, p. 16. Return to text.
- The supposed evolutionary ancestry of birds is still hotly debated by evolutionary scientists. Evolutionist bird expert Alan Feduccia summed up their confusion: ‘The true origin of birds is still up in the air.’ (Cited in Creation 23(2):5, 2001.) Return to text.
- Ref. 1, p. 143. Return to text.