Mystery Of Migration

 

Migration routes of some birds

Migration routes of some birds

When individuals or their propagules (larvae, seeds, and spores, etc.), move from one area to another, this phenomenon is called migration. In stricter sense it implies to periodic two-way movements, to and from a given area and usually along well-defined migration routes (link between two biogeographical regions that permits the interchange of animals and/or plants). Such migratory movements are triggered by seasonal or other periodic factors (e.g. changing day-length), and occur in many animal groups including birds.

Aristotle by Francesco Hayez (1791-1882)

Aristotle by Francesco Hayez (1791-1882)

Perhaps, Aristotle (384-322 BC) was the first man who recognized the seasonal movements of animals, about 2,000 years ago, but the naturalists and other scientists of later period, did not follow the ‘leader’. In his ‘History of Animals’, Aristotle wrote:All animals have an instinctive perception of changes in temperature, and just as men seek shelter in houses in winter, or as men of great possessions spend their summer in cool places and their winter in sunny ones, so also all animals that can do so shift their habit at various seasons.”

If we look at the birds on the basis of migration they can be divided in two broad groups. One is of resident birds that stay close to the general area of their nesting ground and the second belongs to those that migrate or leave their nesting places in winters for warmer places and return again in summers. Among the migrants, there are dispersive birds or transient migrants also that spread often hundreds of kilometers in all direction from their nesting grounds, after the breeding season is over. Their dispersal depends upon the availability of food and even on weather conditions.

Alexander von Humboldt (September 14, 1769 – May 6, 1859) (Artist - Karl Joseph Stieler)

Alexander von Humboldt (September 14, 1769 – May 6, 1859) (Artist – Karl Joseph Stieler)

Till the beginning of 19th century, before migration was verified, it was generally thought that during winters birds (that shift to warm countries) hibernated. It was suggested that they slept through the cold in hollow trees and crevices. The 19th century explorer and naturalist Alexander von Humboldt believed that swallows hibernated during the cold seasons by burying themselves in the mud at the bottom of ponds. Another famous naturalist Gilbert White, Humboldt’s predecessor, was also of the same opinion, a belief that was very common till the beginning of the 19th century.

Now with the combination of hard work and high technology, it has been proved beyond doubt that barring few exceptions, like nightjars etc., birds do not hibernate on change of season, but migrate to climates most suited to them.

Why some species migrate others do not?

After it has been established that many bird species migrate from colder regions to warmer places during winters, a new question has cropped up which is also the biggest mystery of the avian world – why some species migrate others do not? Ornithologists are studying the phenomenon for many years, but still they are not in a position to give any clear-cut answer.

There are several theories on this, but none is able to provide fully satisfactory and exhaustive explanation.

Gilbert White ((18 July 1720 – 26 June 1793)

Gilbert White ((18 July 1720 – 26 June 1793)

1) Short supply of food during winters is considered a major reason for some species to migrate; especially in the cold regions insect-eating birds would not find enough food to subsist on so they leave for greener pastures. For instance, martins and swallows leave their nesting places as insects start becoming scarce.

2) The changing hours of daylight bring about a change in the hormonal balance of birds, which affects the activity of sex organs, and this hormonal change triggers the migratory instinct. This fact is also not entirely true, because the crossbill nests in Europe during the winter season when there is far less daylight.

3) According to one presumption, migratory species had their home originally in tropical and subtropical regions, where they used to breed and raise their young. Over a period of time they over-multiplied and the food became scarce so they set out for more northern parts in search of food for their families, returning south again when the young were fully grown.

4) Another theory, which is exactly opposite to the previous one, says birds had their permanent home in the entire northern hemisphere, but with the advent of the Ice Age they were pushed farther south and were forced even to abandon their breeding grounds. Propounders of the theory argue that with the end of the Ice Age birds again made their way north to their original home and over the millennia, the annual features of migration became an established phenomenon.

Above theories are only a few of the several ones that are afloat, but none of them is able to provide answers to all the questions related to the mystery of bird migration. It seems that there are a number of other causes also which are responsible for the phenomenon.

Originally it was believed that the birds follow a definite route while migrating, but now it has been proved that this is not correct. They travel in a specific direction, but route may differ. In case there is a natural barrier, like high mountain range, then, they may avoid crossing over the peaks, instead they may seek more convenient routes such as mountain passes, etc., or when there is a sea they may not venture over the vast expanse of open sea, but fly over islands where they can take rest if there is any need.  

Salim Ali (Attribution - V. Santharam, Madras Naturalists's Society & CC BY 2.5)

Salim Ali (Attribution – V. Santharam, Madras Naturalists’s Society & CC BY 2.5)

There was also a common belief that birds cannot survive harsh atmospheric conditions at the altitude of 20 to 22 thousand feet. Salim Ali, the doyen of ornithology in India, proved it wrong in 1981 by establishing that even the small birds of the size of starlings, are capable of coping with the difficult conditions at such a height.

Half of the birds that breed in Europe fly south

Migration from the cold regions takes place at a very large scale. Almost half of all the birds that breed in Europe fly south. Some spend their time in southern or south-western Europe, but many winter in Africa and still others journey as far as the tropics or even further up to South Africa. In certain years it has been observed, as many as 300 crores (3000 million) birds journey south, across or around the Mediterranean Sea. In most autumns, it is presumed that, about 4 lakh birds (400 thousand) pass over every kilometers of North Africa’s long coast.

Some European birds, e.g., nightjars, swallows and cuckoos fly as far as southern Africa, a distance of about 10,000 km. They travel twice a year, in spring and autumn. From Scandinavia, chaffinches cross a mere few hundred kilometers of sea to winter in the British Isles. Greenland wheateaters make the longest non-stop flights by any land birds. They fly about 3500 km non-stop from Greenland to Spain. The rufous hummingbird is among the smallest migrants, which makes one of the longest round trips. It can travel as much as 6500 km from its breeding grounds in Alaska to its wintering grounds in Mexico and back. Woodcock, an Indian bird, is believed to migrate from its breeding grounds in Himalayan mountain ranges to the Nilgiri Hills in southern India undertaking a non-stop flight of about 2500 km. As far as the longest journey is concerned, Arctic tern holds the world record.

While on migratory flight the speed of a bird is usually less than that of which it is capable over short distances and it is different for different species, e.g., the chaffinch flies at about 54 km per hour, rook about 48 km per hour, jackdaw about 65 km/h and sparrows up to 80 km/h. There are certain birds that fly about 100 km a day on migration, while the record holders travel as much as 325 to 590 km. a day.

Different species travel at different altitudes. In 1976 a plane and Irish radar station reported, whooper swans near Scotland flying at a height of 8200 metres (world’s highest mountain peak Mt. Everest is 8,848 metres or 29029 feet high). Other birds like rook and jackdaw fly at an altitude of 40 to 100 meters, raptors and starlings about 125m, and 45-100m respectively and other small birds usually up to the height of 110 meters.

While on their journey to wintering grounds, except for few exceptions, birds usually stop to rest, and what is more important to obtain food. In bad weather, especially when there is fog, migrating birds may camp for several days in one place waiting for the weather to become suitable, thus there are occasions when journey may take as much as several months in the case of far off destinations. Most species feed on way, while some do not; they get the energy by utilizing fat reserves, which they have accumulated before setting off. For instance one species of warbler may double its weight before setting off across the sea or desert, or both.

How birds find their way?

Usually birds undertake their journey in flocks together with older and experienced birds that know the way. The most puzzling and mind-boggling fact for scientists is that in some species journey is undertaken alone and that too for the first time and yet they do not go astray. The young of the gannet abandon the nest at the age of about 80 days and plunge into the sea, swimming in the direction of migration until they are able to fly, which is at the age of 100 to 110 days. From the above situation it seems that migrants have an innate sense of long-distance orientation, but how do they navigate and what influences this trait is a million dollar question.  Some of the theories in this connection are:

(1) Memory Theory — For long it was believed that birds find their way from their memory — they simply remember the route. But in order to remember one must have seen it at least once. To check this theory birds were carried in covered baskets or boxes, to far off places and when they were released there they returned to their original place.

(2) Theory of Spirals — After the “memory theory” exploded, a new one appeared. It suggested that birds fly to their wintering grounds and back not along a straight line, but in spirals or circles. These circles gradually expand until the birds see familiar signs, which help them to take up the right course. The champions of this theory were able to adduce some facts in support of it i.e. birds have excellent eye-sight and they can spot material and locations from far away. More so, as it has been established that from an altitude of 200 meters, a bird sees in the radius of 50 km, and from two thousand meters its radius of vision expands to 160 km.

However, this theory too was rejected as it was observed that the long duration of migration flights were accounted for by very different reasons: during flights birds make long stops to rest, feed and wait out stormy weather. While flying over the ocean, birds cannot see any landmarks. Finally, a bird can recognize such landmarks only during repeated flights, while in some species young birds fly for the first time in their lives and that too without any guide and still reach their destinations.

(3) Guided by the old birds — Later, the “Theory of Spirals” was replaced by one according to which young birds are guided by old ones. It persisted for long, but later it was proved that in a number of species young birds do not need the guidance of old ones, e.g. young starlings leave for wintering grounds before the older birds; young cuckoos, who do not know their true parents at all, migrate to the same place where old cuckoos go.

(4) Thermal orientation — There is another theory, which suggests “thermal orientation” to be the reason. It holds that, slight waft of warmth emanating from the south help birds to orient themselves, but it also failed to find confirmation.

Dr. Gustav Kramer

Dr. Gustav Kramer

(5) Sun, Moon and Stars — This theory says birds take help of sun, moon and stars. It was substantiated by experiments with many species of birds. A German ornithologist, Gustav Kramer, conducted a series of experiments with starlings by method of a “circular cage” and proved that birds orient themselves with the help of sun’s position. When the apparent position of the sun changed with the help of mirrors the starlings made corresponding changes in their position.

This is similar to the way in which a sailor uses the sun to establish his location and the course he needs to take for home. For the purpose, he requires two basic facts: the position of the sun in the sky and the position of the sun at the same time in relation to some fixed point elsewhere on the earth – say the Greenwich meridian. Now the sailor can easily obtain the first piece of information just by looking in the sky above him, but for other information human navigators require chronometers and charts.

Birds seem to remember what the position of the sun would be in their home territory at any given time. And just like the sailor they then compare this with the position of the sun as they see it. With their internal physiological chronometers they mark time at a steady pace, irrespective of conditions outside the body, and these they use to clock the sun.

By using their internal clock and following the motion of the sun, the migrating birds could know where they are in relation to home territory and therefore the direction they must fly in to reach that destination. In the case of night-flying migrants similar systems have been postulated in which the moon and the stars are used instead of the sun.

Birds can orientate themselves by the position of the sun, even when the sky is overcast, but not in very foggy conditions. When light enters the Earth’s atmosphere during the cloudy conditions, there is oscillation in light at all angles, but as it continues through the atmosphere it may become polarized and the oscillation of light rays restricts in the same direction. This light is beyond human perception. The extent and angle of polarization depends on the sun’s position in the sky. Birds use the pattern of polarized light as a navigational tool for locating the sun’s position.

Birds that migrate in night also use this light. When they start their journey in the evening, this light helps them in ensuring that they head off in the right direction even though the sun has already set. With the setting of the sun a band of maximum polarization runs across the sky from north to south. If the sky is clear, this is the most authentic and dependable guide. In spring in the northern hemisphere, birds head north by flying parallel to this band, using some form of biological clock and the position of the sun to orientate themselves.

It is believed that at the nesting site and in the surrounding neighbourhood, birds find their way mainly by their memory of various landmarks. This does not mean that resident birds do not have orientation capability. Birds captured from their nests and released several hundred kilometres away returned to their home site, yet they couldn’t possibly know the countryside such a distance from their nests.

In an experiment one starling was taken 341 km north of its nest in a plane. When it was released, it came back to its home within few days; it was captured again and this time it was taken 500 km south of its nest, to utter surprise of the scientists it came back within five days.

In another experiment some swallows were taken 35 km away from their nests and they took 2.5 to 8 hours to return. Why these birds took such a long time to cover such a short distance? The reason, perhaps, is that in many places swallows are resident birds (remain in their nesting ground throughout the year) and usually find their way by their memory of various landmarks, but when they were taken far away from their home, they no longer knew the landmarks and consequently, they had to orientate themselves by the position of the sun for which they had to take longer route so that they can adjust the direction of their flight. Once they had reached the ground with the familiar landmarks they had no difficulty in finding their home site. This proves that the long-distance orientation is not limited to migratory birds, but is developed in resident birds too.

In the case of night flying migrants it is believed that they establish their position from stars, which is not an easy task, because the position and the pattern of stars changes with longitude and season. In northern hemisphere Pole Star is the only fixed and constant star and during the course of night the rest of the stars appear to rotate round it. Believers of the “star guide theory” say that somehow birds can relate to it and use the position of the Pole Star to navigate. While returning from their wintering grounds birds are usually under pressure from their breeding instinct and they travel faster towards their nesting sites. This journey can be three times faster than the previous one.

(6) Infrasounds help in their migration — It has been suggested quite recently that the ability of birds to hear infrasounds help in their migration. These sounds are below the hearing range of human beings (usually 20 to 20,000 Hz); however, they are often ‘experienced’ as vibrations. For instance, the sound of wind humming around tall buildings or passing through canyons, waves crashing on shores, have very low-frequency components that travel vast distances and can be heard by birds flying at far off places.

Some scientists are of the opinion that migrating birds pick up these infrasound signatures of landmarks on their route and construct a sound map in their mind. Although, it is not a very precise map, it does provide a reassuring back- up for other sensory systems. 

(7) Theory of magnetic orientation — In the mid-nineteenth century a Russian scholar A.F. Middendorf came out with the “theory of magnetic orientation”. According to it birds orient their migration routes under the influence of earth’s magnetism, a series of lines of force that rise at angles to the Earth’s surface – they are almost vertical at the poles but parallel to the ground at the equator. Believers of this theory say that instead of discerning north and south directly, as a compass does, birds sense their position from the angle at which the magnetic field intercepts the ground.

To verify this theory, many experiments were carried out, in which artificial magnetic effects were created to interfere with the bird’s orientation, but birds exploded this theory too. During the course of the experiment it was also proved that earth’s magnetic field has only a slight influence on animals.

In the recent years Theory of magnetic orientation has again gained ground. Results of new researches have once again revived the belief that it is the Earth’s magnetic field that helps birds in navigating through the alien lands while migrating. Although, they do not have any equipment like maps, GPS or compass but it seems they have inbuilt ‘navigational devices’ that make them “perceive” Earth’s magnetic field.

Recent studies indicate that part of birds’ optical system could be affected at a molecular level by weak magnetic fields. Birds while migrating appear to obtain from the Earth’s magnetic field visual information about whether they’re rightly oriented along the migration route. The birds aren’t merely pointed in a specific direction by the magnetic effect, like flying compass needles. They in fact see the field in some way. Scientists don’t know precisely what the birds see, but their research says that they get some sort of visual feedback when they’re on path and a different cue when they stray, kind of like clear reception when a TV antenna is aligned vs. static when it’s not. It is likely that the birds blend this information with other navigational cues.

Other animals, like salmon, salamanders and hamsters, likely have similar abilities. Researchers traced the system to “receptors” in the brain and eyes, proteins that absorb light & provide information about day, night and color. The retina comprises of black & white receptors, color receptors, and another type known to regulate circadian rhythms. Receptor that makes us sleepy at night also appears to play a role in processing magnetic signals. Magnetic reception has already been observed previously by the scientists. For instance, the “magnetoreception” of bacteria filled with magnetic particles makes them natural compasses. But the way it’s accomplished in higher animals, whose systems aren’t chock-full of tiny magnets, has been a mystery. Recent studies do not settle the question, but do provide a promising avenue for further inquiry.

Magnetoreception

Birds use several sources of directional information for navigation, for example, the time-compensated sun-compass of homing pigeons and diurnally-migrating songbirds. Nocturnally-migrating birds cannot use the sun, but use the stars for orientation. Additional information, like the glow in the western sky after sunset caused by the setting sun can be used shortly after sunset along with the stars. However, while taking the help of the sun, stars, and the glowing sky it requires that at least part of the sky is visible to the birds. Many of them are also able to accurately orient when the sky is not visible (e.g., cloud cover). This requires non-visual sources of information. Several studies have established that birds are susceptible to the Earth’s magnetic field. For instance, Pigeons and many other birds use the geomagnetic field as a compass, and are also receptive to slight temporal and spatial variation in the magnetic field that are potentially useful for determining location.

The avian magnetic compass may be:-

  • a light-dependent, wavelength-sensitive system that functions as a polarity compass (distinguishing poleward from equatorward rather than north from south). The receptor is within the retina and is based on one or more photopigments, perhaps cryptochromes.
  • based on magnetite and might serve to transduce location information independent of the compass system. This receptor appears to be linked with the ophthalmic branch of the trigeminal nerve and is receptive to very small changes in the intensity of the magnetic field.

The inclination compass — Rather than using the polarity of the earth’s magnetic field (i.e., the compass points north), birds use the inclination of magnetic field lines relative to gravity. An inclination compass offers information about the axis of the field lines as well as the direction towards the magnetic pole or equator. The intersection of the magnetic field lines with the horizon points polewards (in both the northern and southern hemispheres) and the direction where the inclination angle diverges points toward the equator.

Light-dependent magnetic compass — External magnetic fields can influence photon-induced processes that involve biomolecular reactions, recent work suggests that molecules called cryptochromes are involved. Magneto-sensitive photoreceptors dispersed all over the retina may show increased or decreased responses to light, depending on their alignment relative to the magnetic field and allow birds to actually see the magnetic field lines. Current evidence indicates that the right eye of birds is more important than the left in ‘seeing’ magnetic field lines. Using an inclination compass can cause problems close to the magnetic poles (90 degrees) and at the magnetic equator (0 degrees). The alignment of the field lines (vertical at the poles and horizontal at the equator) makes it impractical to select the correct direction. Fortunately for birds, they have other means also of obtaining directional information (e.g., sun and stars).

Earth’s magnetic field to navigate — Latest evidences suggest that birds use light-induced magnetically-sensitive chemical reactions (chemical magnetoreception) to orient themselves, but no chemical reaction has been shown to respond to magnetic fields as weak as the earth’s. Some scientists have now synthesized a molecule (carotenoid-porphyrin-fullerene, or CPF) that is sensitive to both the magnitude and the direction of magnetic fields as tiny as the Earth’s (which is, on average, one-twenty thousandth as strong as a refrigerator magnet).

How could this impact the direction taken by a migrating bird? Birds seem to orientate at dusk, and cryptochromes (light-sensitive proteins known to be present in the retinal neurons of some birds) form their pair of free radicals when “activated” by the blue light typical of dusk. Thus, dusk might set in motion the birds’ magnetic sense, producing the radical pair. The concentrations of each free radical would be controlled by the Earth’s magnetic field that varies with latitude. As a result, the radicals would bind in varying degrees with other signalling molecules, depending on how far north or south the animal is. How would birds decode this “magnetic sense?” Some investigators believe that birds have an additional layer to their vision that, when switched on, allows them to visually “see” the Earth’s magnetic field in a manner similar to “head-up displays” in fighter jets and some cars, where transparent screens displaying information are built into windscreens.

Mechanism of magnetoreception — Migrating birds remain on their path because of chemical reactions induced by the Earth’s magnetic field. The birds are receptive even to fast fluctuating artificial magnetic fields. These fields had no effect on magnetic materials such as magnetite, indicating that the birds do not depend on simple chunks of magnetic material in their beaks or brains to decide direction, as experts had earlier suggested.

Homing pigeon Navigation -- (Graphic Source - London Evening Standard)

Homing pigeon Navigation — (Graphic Source – London Evening Standard)

Homing pigeons orientate with the help of their beaks — Although it has long been known that birds have the capability to use the Earth’s magnetic field for navigation, but how it is done has not yet been explained.  However, a team of German scientists, led by Gerta Fleissner discovered that homing pigeons possess iron-containing structures in their beaks, which allow them to analyze the earth’s magnetic field – much like a compass. To be more precise, iron-containing subcellular particles of maghemite and magnetite were discovered in sensory dendrites of the skin lining the upper beak. Unlike the earlier premise concerning magnetoreception that assumed magnetite as the vital mineral, now it seems that both iron minerals are necessary for a sensitive sensory system that functions as a magnetometer. Through the signals picked up, the birds can work out where they are and set out on the best course home.

The dendrites in the pigeon bill are positioned in an intricate three-dimensional pattern with different spatial orientation designed to analyze the three components of the magnetic field vector separately.  They respond to the Earth’s external magnetic field in a very sensitive and precise manner, thus acting as a three-axis magnetometer. Thus, the birds may sense the magnetic field independent of their motion and posture and can distinguish their geographical position. The amazing abilities of homing pigeons made them invaluable during both world wars, with both sides using them to send messages over enemy lines. Thirty-two of the 250,000 pigeons used by UK forces in World War II were even awarded medals for valor. Scientists are still discovering more about the incredible abilities of the pigeon. It has also been found that they can memorize 1,200 pictures.

However, despite such impressive memories, pigeons are not the most intelligent birds, according to researchers. A team in 2005 judged the intelligence of a range of birds and concluded that crows, rooks, jays and ravens topped the IQ league, while the New World quail earned the dubious honor of being the most stupid.

Like pigeons, many other migrating birds too display an extraordinary ability to fly thousands of miles to return to the same garden or a tree year after year. Scientists believe they may also be containing similar iron-containing cells in their beaks. Fleissner pointed out similar iron-containing cells had been found in the beaks of warblers, chickens and robins and so it may well turn out to be the way that other species also navigate. “We expect that the pigeon-type receptor might turn out to be a universal feature of all birds,” she said.

In the past, scientists have suggested that the birds use position of sun and stars to find their way, although in 2004 researchers found that many of them follow roads rather than their internal compass to plan their route. According to the recent findings by Italian scientists birds can create ‘odor maps’ of areas they fly over, which may help them find their way.

Flight Formations

Many long distance migrants, like geese, cranes etc., travel in V-formations, which pleases many observers on the ground but, very few people know that there is more to this flight formation than just its beauty. The fact is that by flying in this way birds not only save their precious energy, but also get a push-up in the vacuum in the air created by their colleague flying in front. In V-shape flying, they are also able to keep an eye on their leader.

As a bird flies, the flapping of wings produces small circular movements of air or eddies on the tips, which hampers the forward movement of the producer because it acts as a drag on its flight. But, the bird immediately behind it gets a boost because eddies provide extra lift and the particular bird does not have to work hard to remain airborne. Now the question arises, in such a situation, the leader will always be at a disadvantage being at the forefront and receiving no help? To solve this problem birds keep on changing their position every now- and- then by dropping back in rotation and allowing others to take the lead. This system gives every bird a chance to get a clear view of the route.

According to the latest study (the Nature, January 2014), while flying in a “V’’ formation birds also choreograph flapping of their wings with exquisite precision to help them in their flight while migrating to distant places. This is the conclusion of scientists who tracked a group of large black birds, each equipped with a tiny GPS device to record its position and every wing flapped.

Scientists have long theorized that birds adopt “V’’ formation for aerodynamic reasons. When a bird flies, it leaves a wake. The idea is that another bird can get a boost from an updraft of air in that wake by flying behind the first bird and off to the side. When a group of birds use this trick, they form a “V.”

It’s been difficult to study this in the wild. Researchers from the University of London’s Royal Veterinary College and elsewhere studied 14 ibises as they migrated between Austria and Italy. The birds had been taught to follow an ultra light aircraft as part of a conservation program.

An analysis of a seven minute period showed that when the ibises flew in a “V”, they positioned themselves in just the right places to exploit the updraft in another bird’s wake, which lets them conserve their energy.

They also appeared to time the flapping of their wings to take full advantage of that updraft, by making a wingtip follow the same undulating path through the air as the wingtip of the bird up ahead.

And when one bird flew directly behind another instead, it appeared to adjust its flapping to reduce the effects of the wake’s downdraft. So birds can either sense or predict the wake left by their flock mates and adjust their flapping accordingly, a remarkable ability, the researchers said.

This is the first experimental evidence that birds can adjust their flapping to take advantage of the wake.

Birds too make mistakes while migrating

Experts have found that migrating birds too make mistakes in terms of direction. It is usually believed that many vagrants land up at wrong wintering areas because they get blown off course by the storms and gales, but the new study suggests probably it’s a result of an error in the genes that govern their migratory response.

Research team of experts from the Ornithological Society in Bavaria, the Helmholtz Centre for Environmental Research (UFZ) and the University of Marburg concluded after assessing several thousand reports of Asian birds that had strayed to Europe.

They observed that the distance between the breeding places in northern Siberia and the wintering grounds in southern Asia was often similar to the distance between the breeding grounds and Europe. Their study showed the vagrants to have been seen by birdwatchers in Central Europe were long-distance migrants from Far East Asia.         

Scientists claim the more similar the distances, and more numerous a particular species, the higher the possibility of the species straying to Europe. They evaluated both the distances, between the breeding area and the wintering ground and between breeding area and Central Europe, and also measured the wingspan, the body mass and the size of breeding area for 38 species of migratory birds of which eight were from leaf-warbler family and six from the thrush family. No correlation between the body size and the frequency of vagrants was found. However, an interesting aspect which has come to light is that the species most likely to come to Europe were those that are widespread in Asia, and were as common there as their relatives the Chiffchaff and European Willow Warblers are in Central Europe.

The most often spotted species was Yellow-browed Warbler (Phylloscopus inornatus). Voluntary ornithologists in Central Europe reported about it around a thousand times between 1836 and 1991, while other Asian leaf-warblers were seen much less frequently, if at all, in Central Europe. Five thrush species were reported nearly 100 times.

Another argument that confronts and challenges the older belief is that had the vagrants been blown off course by the storms and gales, the smaller species should have been affected more frequently than the larger ones.

Experts say, “The more plentiful a species is, more the likelihood that one of them will be ‘erroneously programmed’ and go astray. Although they fly the same distance but in the reverse direction that takes them to Europe. This is why we have relatively large numbers of vagrants from Asia here.”

When asked, what makes them suspect the genetic migratory program of birds they say the flight direction and duration are passed on from one generation to the next, which means that migration is the result of a genetic program that enables birds to adjust to environmental conditions.

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