The orientation of migratory birds

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Birds face many challenges during their migrations, not least orientation. How do they set a course and stay on it, what landmarks do they use to navigate between regions that can be almost 10,000 km apart without getting lost? Numerous experiments have shown that birds use a variety of methods to position themselves on the “mental map” of their journey: stars and celestial bodies, to which they orient themselves using an “internal compass”. They also make use of geomagnetism, which remains the only way to orientate themselves when all visual reference points are unusable. An ‘internal calendar’ allows birds to change course at the right time according to the spatio-temporal conditions of their route. Although orientation mechanisms are genetically determined, birds show a certain behavioural plasticity thanks to their receptivity to a wide range of geographical and meteorological information.

1. Basic principles

Migratory animals face many challenges as they move between their departure and arrival points, of which navigation and wayfinding are among the most serious. In the case of birds, how do they define and maintain, even in difficult weather conditions, day and night, the migration vector that best links their departure and arrival areas, which can be almost 10,000 km apart? (See The flight of birds).

Figure 1. Migrations of the northern wheatear (Oenanthe oenanthe) between the vast Palearctic region and the extremity of the Nearctic region (from Greenland to Siberia and Alaska) on the one hand and Africa on the other. [Source scheme © by the author]
Navigation involves setting a course in a given direction and following it faithfully from the point of departure to the point of arrival, regardless of the hazards encountered along the way. Things get more complicated when geography makes it necessary to change course along the way. Selecting a heading and maintaining it over long distances involves different mechanisms. Figure 1 illustrates the migration of the northern wheatear Oenanthe oenanthe, a journey of more than 10,000 km that involves the entire population of birds breeding in the vast Palaearctic region, including Iceland and Greenland. Even the populations that nest in Alaska spend the winter in Africa, having colonised Alaska from Siberia.

The fact that very young birds set out on long migrations on their own, independently of their parents, from the first year of life, just a few months after birth, shows that orientation mechanisms are innate and therefore genetically determined. In particular, they have a kind of ‘internal calendar‘ and ‘internal compass‘. These characteristics automatically dictate the spatial and temporal patterns of the birds’ journeys to their wintering grounds in the autumn and back to their breeding grounds the following spring.

Figure 2. Barn sparrow (Hirundo rustica). [Source : Photo © J. Blondel]
Certain performances, in particular fidelity to the breeding site, can be staggering: using rings that allow the same individual to be tracked, it has been shown that the barn swallow Hirundo rustica (Figure 2) moves between the same wintering and breeding sites with an accuracy of a few metres over the seasons. For example, one barn swallow made the same journey from South Africa to its nest in a stable in Germany for three years in a row!

Such orientation skills have been demonstrated many times in groups as diverse as passerines, waders, ducks, storks, birds of prey and many others. Most small migratory passerines fly alone or in loose groups, especially nocturnal migrants. They usually take off at nightfall and fly non-stop until the next morning, covering 300 to 600 kilometres in one go at a speed of around 40 to 50 km/h. Birds use several tools to find their way, but they all rely on one basic tool, the magnetic compass, which is effective in all weathers and is used to calibrate the others, particularly the stars (sun, moon, stars).

2. Orientation mechanisms

It was not until the 1950s that birds’ orientation mechanisms became the subject of in-depth research. This area of research required experiments that were difficult to set up, which meant formulating clear research hypotheses that could be validated or refuted experimentally. This was done using very special equipment. It was at this point that remarkable discoveries were made about the mechanisms of orientation, which turned out to be numerous and complementary, each used according to the circumstances.

2.1 Solar compass

Figure 3. A, orientation cage used to measure the orientation ability of migratory birds. Numerous perches connected to electronic circuits are arranged around the cage. Each time the bird lands on one of them, the impulse of the chosen direction is recorded (B, arrows). [Source: EEnv scheme]
The first logical source of information for the researchers was the position of the sun in relation to the migration axis. The sun has the advantage that

  • It is visible, except at night or on overcast days,
  • It occupies a predictable position in the sky depending on the season and time of day.

This is something that birds can take advantage of as soon as they have a sense of time.

Indeed, empirical evidence, largely confirmed by orientation cage experiments (Figure 3) conducted by the German biologist Gustav Kramer, shows that the position of the sun is widely used as a directional cue by diurnal migrants [1].

Starlings Sturnus vulgaris kept in orientation cages (see Figure 3) took the appropriate direction at the time of migration departure and this direction changed when the operator changed the origin of the light source simulating sunlight using a mirror (Figure 4).

Figure 4. Experiments showing, with starlings Sturnus vulgaris kept in orientation cages, the impact of the origin of the light source simulating sunlight using a mirror. A: Normal condition: the sun is in the east, and the birds take the appropriate direction – north-west – when they start to migrate; B: A mirror deflects the light source by 90° towards the south, so the direction of migration is now south-west; C: The mirror deflects the light by 90° towards the north, so the direction is now north-east. In each condition, the number of times the bird lands on the perches is measured, and the impulse of the chosen direction is recorded (A2, B2, C2). [Source: EEnv scheme]
Figure 5. Birds adjust their flight direction according to the position of the sun in the sky over the course of a day (from sunrise to sunset) to reach their destination. gN, true north. [Source: EEnv scheme]
Using the sun implies that the bird adjusts its flight direction according to the position of the sun at different times of the day (Figure 5), which was again demonstrated experimentally on captive birds using an artificial sun simulated by powerful electric lamps.

The fact that azimuth changes throughout the day is another way of testing the extent to which birds use a genetically determined solar compass. To test this solar compass hypothesis, we can shift the bird’s internal clock by a few hours by keeping it in a dark aviary for a few weeks:

  • Firstly, the bird’s internal clock can be disrupted by exposing it either to continuous light or darkness, or to sequences of alternating light and darkness of different durations.
  • Secondly, the room in which the captive bird was kept was subjected to a 24-hour day/night cycle, but 6 hours out of phase with the natural cycle.

This series of experiments showed that [2] :

  • If the bird’s clock is advanced or delayed by 6 hours, the bird will fly in the wrong direction by 90° in either direction, showing that the bird uses an ‘internal solar compass’ to determine its direction.
  • If the bird’s internal clock is advanced or delayed by 12 hours, the bird will fly 180° in the opposite direction, heading north in autumn when it should be heading south.

When the sky is overcast and the sun is useless, the bird does not stop its migration but uses other means, in particular the Earth’s magnetic field. We will see later that this field is the only source of information that can be used by the bird’s compass.

While the sun is widely used by diurnal migrants such as white storks (Ciconia ciconia) and most large birds of prey, it is of no help to most small trans-Saharan migrants. The latter, especially the several billion passerines that migrate seasonally between Eurasia and sub-Saharan Africa, travel at night and must therefore use other means of orientation.

2.2 Night-time compasses: the moon and stars

A number of excellent experiments using artificial skies that can be rotated in planetariums have demonstrated the role of constellations in the orientation of migratory birds. The first conclusive studies on nocturnal migration were carried out by physiologists. By keeping migratory birds in an orientation cage, in this case eurasian blackcap Sylvia atricapilla, they discovered that when the birds were about to set off on their migration, they were subject to intense and uncontrollable agitation – the so-called “Zugunrhue” [3]. This particular behaviour occurs at dusk twice the year, the first time at the beginning of spring migration, and the second at the beginning of the fall migration.

Figure 6. View of the Milky Way over the Ecrins (Alps, France), taken from the Galibier pass, September 2021. Migratory birds are able to orient themselves thanks to the stars and constellations.  [Source photo © Alain Herrault, reproduced with the kind permission of the author].
From these findings, researchers built planetariums equipped with artificial skies to experimentally test their hypotheses on nocturnal orientation. It was in the 1950s that the German ornithologist Franz Sauer[4] discovered that birds are capable of orienting themselves with the help of the moon and stars (Figure 6).

Star navigation, which is necessarily nocturnal, requires that :

  • the bird must be able to recognise the azimuthal position of a star or constellation in relation to the trajectory it must follow ;
  • the position of the reference star or constellation is associated with a notion of time, so that the bird can adjust its flight path according to the movement of the star constellations during the night (Figure 7).

Figure 7. Movement of the stars around the North Star above Mont-Aiguille (Vercors, France). Migratory birds adjust their flight paths according to this movement during the night. [Source photo © Alain Herrault, reproduced with the kind permission of the author].
The amount of compensation required depends on the position of the reference constellation in relation to the celestial equator.

The hypotheses associated with stellar orientation have been tested experimentally in a planetarium. For example, a situation can be created whereby the bird’s internal perception of time is offset from real astronomical time by advancing or delaying the position of the stellar configuration in an artificial sky constructed in a planetarium. If the bird’s internal chronometer (its sense of time) is coupled to the stellar configuration, we should observe corresponding changes in the migratory orientation taken by the bird in its orientation cage. This is indeed what Stephen T. Emlen has demonstrated in a series of beautiful experiments carried out in a planetarium equipped with a removable artificial sky (Figure 8). During periods of migratory activity, the birds positioned themselves according to the arrangement of certain constellations and modified their position as the experimenter rotated the sky to remain aligned with the reference constellations. So much so that the bird could be made to somersault when the planetarium sky was rotated 180°!

Figure 8. The Flint Michigan planetarium with orientation cages used by Stephen T. Emlen to study the orientation taken by the bird according to the constellations visible in the sky. [Source photo © Stephen T. Emlen, courtesy of the author]
These experiments also showed that, just as homing pigeons use a magnetic compass to calibrate their solar compass, passerines such as the garden warbler Sylvia borin or the european robin Erithacus rubecula use their magnetic compass to locate the axis of rotation of the night sky.

In addition, migratory birds regularly recalibrate their night compass by reference to the magnetic compass in order to take account of changes in the configuration of constellations as the migratory bird moves south in autumn and north in spring.

A further complication for star-based night navigation is that trans-Saharan migrants, whose destination is far away in South Africa, are confronted with completely different star configurations as they cross the equator and change hemispheres (Figure 9). Their orientation system involves knowing, and therefore ‘reading’, the new star configurations to which the birds orient themselves.

Figure 9. A map of the sky showing the brightest stars in each of the northern (left) and southern (right) hemispheres. When they change hemisphere, the birds’ orientation system involves “reading” the new stellar configurations. [Source Manuel Strehl, CC BY-SA 2.5, via Wikimedia Commons]

2.3 The Earth’s magnetic field

The first studies of bird migration using radar in the 1950s showed that nocturnal migratory flows are oriented in the right direction even when the sky is completely overcast, making stellar navigation impossible. In 1955, the German ornithologist Gustav Kramer hypothesised that bird navigation involves two stages:

  • The first is based on a ‘mental map‘, in which the geography of the migration route to the destination is imprinted;
  • The second, using a ‘compass‘ inscribed in the bird, which enables it to determine the route to take to reach its destination.

Using ‘orientation cages’ that he built to test his hypotheses, Kramer discovered that ‘premigratory bulimia’ is the prelude to ‘migratory activity ’proper. Pre-migratory bulimia is a frenzied eating behaviour that consists of consuming large amounts of food. The food, usually berries that are rich in carbohydrates, is converted into fat, which is used as fuel during the long migratory flights. At this point, the birds Kramer has in captivity, in this case garden warblers, automatically head in the right direction, i.e. south-west in autumn and north, then north-east in spring.

Although the results were sometimes contradictory, other experiments carried out in the same conditions with robins showed that the birds were able to orient themselves correctly – south-west in autumn, north-east in spring – even when the orientation cage was placed in a dark room where the sky was invisible. The fact that these birds, kept in cages where they could neither fly nor see the sky, were able to orient themselves in the right direction suggested that the orientation mechanism was based on the existence of a genetically determined magnetic compass. Taking the experiment a step further, Kramer’s team experimentally modified the Earth’s magnetic field using magnetic coils and magnets to change the apparent direction of magnetic north. The birds responded as predicted to this new orientation, providing irrefutable evidence for the importance of the magnetic field (Figure 10) [5] in bird orientation. Indeed, it is likely that geomagnetism, as a source of genetically determine information, is the best source of information, the only one left when all others cannot be used [6].

Figure 10. Earth’s magnetic field. A, Diagram showing how the field lines (represented by arrows) intersect the Earth’s surface and how the angle of inclination (the angle at which the field lines intersect the Earth’s surface) varies with latitude. At the magnetic equator (the curved line that crosses the Earth), the field lines are parallel to the Earth’s surface. The field lines become progressively steeper as one moves northwards towards the magnetic pole, where the field lines point straight down into the Earth and the angle of inclination is 90°. The intensity (strength) of the field varies in a slightly different direction to the tilt; the intensity is strongest near the magnetic poles and weakest near the equator. B, Diagram illustrating four elements of the geomagnetic field vectors that could, in principle, provide animals with information about their position. The field present at each location on the Earth is defined by a total intensity and an angle of inclination. C, The tilt isolines (in red) are represented in increments of 10°. Over a large part of the globe, inclination is strongly correlated with latitude and can therefore be useful in a magnetic map. The total field strength isolines (in blue) are represented in increments of 5 µT. [Source: diagram adapted from Lohmann et al. Magnetic maps in animal navigation, ref. [4], published as licence Creative commons 4.0 ].

2.4 Mental maps

The mental maps referred to by Gustav Kramer enable the bird to position itself in a geo-topographical space on which its points of departure and arrival are marked. Once this positioning has been achieved, the bird uses its “directional compass” (internal clock) to maintain its course throughout its journey.

Whatever the orientation mechanisms used, day and/or night, the migratory journey takes place in geographical areas characterised by a series of visual, olfactory or even solar cues, as some studies suggest. Hence the idea that birds have ‘mental’ or ‘cognitive’ maps to guide them, which appear as mosaics of landmarks visible from afar.

Figure 11. Bretolet Pass barred with mist nets designed to capture migratory passerines. Ornithologists conduct long-term studies of bird migration here.  [Source photo © Lionel Maumary, reproduced with kind permission].
To move from landmark to landmark, the migratory bird uses its magnetic, solar or stellar compass. Little is known about the nature of the visual landmarks on which the mental map is built, but we can assume that geo-topographical features such as mountain ranges, river corridors, valleys, passes and coastal strips play an important role. It appears that a series of successive landmarks are used in turn along the bird’s route, particularly during long intercontinental journeys. This is suggested by observations of migratory flows as seen by radar or simply by visual observation. The success of ringing camps for migratory birds, such as those set up at the Bretolet Pass on the border between France and Switzerland in the Alps (Figure 10), is a good example. Landscape features that are of no use for nocturnal migrants take on their full significance at the end of the migratory journey, as the birds approach their final wintering or breeding destination, depending on the time of year.

The combination of a mental map and a compass (stellar or magnetic) enables the bird to navigate efficiently and safely in all weather conditions. Radar observations have shown that wind-driven migratory birds are able to reorient themselves and return to their original direction once the turbulence has passed. This is likely to happen when birds encounter a front of the mistral, a strong north/north-westerly wind, during their return migration between Africa and the French Mediterranean coast in spring. Birds can also make profit of the wind during their migration, as shown by  massive migrations flows observed when wind conditions are favourable.

2.5 Positioning in time and space

Detailed studies of one of the great trans-Saharan migrants of the European avifauna, the garden warbler, have shown that two genetically integrated programmes are closely linked:

  • a temporal programme, which guarantees the correct orientation for a given length of the migratory journey;
  • a spatial programme that guarantees the appropriate direction for the birds to follow, a direction that may change along the way.

Figure 12. Change of direction of migrants (here Phylloscopus trochilus) in Europe. [Source: author’s scheme]
The existence of these two programmes is clearly demonstrated by the migration system between Eurasia and Africa. Eurasia is home to ca five billion birds, more than a third of which are trans-Saharan migrants. For example, populations of the tiny (less than 10 g) willow warbler Phylloscopus trochilus, which nests throughout Eurasia to the edge of the Siberian taiga, must follow a north-east to south-west course of about 9,000km to reach the European peninsula. Then, when they reach the edge of western Europe, on the Iberian Peninsula, they will have to change direction abruptly and head north-south towards Africa (Figure 12). This sudden change of direction has been demonstrated experimentally in orientation cages, where garden warblers, whose migratory system ithe same as that of the willow warbler, fly in a north-east-south-west direction during the first part of their migration, and then ‘automatically’ and suddenly fly in a north-south direction to begin the second part of their journey. The behaviour of these warblers in captivity in central Europe, where the experiments took place, provided evidence that the directional vectors and their change at the end of a predetermined period constitute a ‘pre-programme’ genetically determined in the bird’s internal memory.

3. Human use of birds’ orientation skills

Figure 13. Monument to the Soldier Pigeon commemorating the role played during the First World War by Belgian pigeon fanciers and their carrier pigeons. [Source: photo © EmDee, CC BY-SA 3.0, via Wikimedia Commons]
The ability of birds to find their way over long distances has been valued by humans since immemorial times, as we know that ‘postal pigeons’ were used as far back as Ancient Egypt. Closer to home, in many countries, including the United States, the craze for carrier pigeons led to competitions in the 19th century. But it was during the two world wars of the 20th century, particularly the First World War, that pigeons were used as messengers. So much so that a monument to the ‘soldier pigeon’ was erected in Brussels to commemorate the important role played by these birds (Figure 13).

4. Messages to remember

Migratory birds have a navigation model consisting of a set of genetically determined mechanisms in a rigid and stereotyped sequential order.

  • These mechanisms are based first and foremost on the existence of a mental map illustrating the route to be taken.
  • The orientation process itself is based on the existence of compasses that define the direction to take in relation to astronomical landmarks, the stars of the day (the sun) or the stars of the night (the moon, the constellations).
  • In addition to these reference points, there is also a magnetic compass, which is used in addition to the others, or exclusively when the others are unusable (night navigation on a cloudy day).
  • To demonstrate the components of this model, we need to carry out experiments in orientation cages.
  • Migratory birds know how to interpret the information provided by the wind, as demonstrated by the fact that migration is particularly massive when wind conditions are favourable.
  • Different orientation systems work in synergy.
  • The various steps of the migration process are based on a circannual time programme, a kind of genetically determined internal clock that “tells” the birds what to do at each of the main stages of the annual cycle: reproduction, moulting, migration, etc.

    Notes & references

    Cover image. Flight of migrating grey cranes [Source: Photo © J. Blondel].

[1] Kramer, G. 1952. Experiments on bird orientation. Ibis 94, 265-285.

[2] All these laboratory experiments were conducted according to the rules of the experimental approach, which require the movements of experimental birds to be compared with those of ‘control’ birds not subjected to the tests.

[3] Zugunruhe, also known as ‘migratory restlessness’, is a feeling of anxiety that occurs in migratory animals, particularly birds. Even when they are locked up without being able to see the sky, birds display this behaviour during the seasons when they are due to migrate.

[4] Sauer, F. 1957. Die Sternenorientierung nächlich ziehender Grasmücken (Sylvia atricapilla, borin und curruca). Z. Tierpsychol.14, 29-70.

[5] Lohmann, K.J., Goforth, K.M., Mackiewicz, A.G. et al. Magnetic maps in animal navigation. J Comp Physiol A 208, 41–67 (2022). https://doi.org/10.1007/s00359-021-01529-8

[6] Wiltschko, R. & Wiltschko, W. 1972. The magnetic compass of European robins Erithacus rubecula. Science 176, 62-64.

 


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To cite this article: BLONDEL Jacques (January 17, 2025), The orientation of migratory birds, Encyclopedia of the Environment, Accessed February 19, 2025 [online ISSN 2555-0950] url : https://www.encyclopedie-environnement.org/en/life/orientation-migratory-birds/.

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