As migration is called the annual flight of migratory birds from their breeding grounds to their wintering grounds and back again. It is estimated that there are 50 billion migratory birds around the world every year, around five billion of them between Europe and Africa .
As a migratory one is bird then called when several seasons spent in different places. Obligatory migratory birds always leave their breeding areas at roughly the same time and regardless of climatic conditions, fly to their winter quarters on roughly the same routes and return in the following spring.
The counterpart to the migratory bird is the stationary bird . Bird species in which only a part of the population migrates are called partial migrants . Another hybrid form are line birds : They leave their breeding area in winter, but remain in the same latitudes .
Features of bird migration
Distances and routes
- Many migratory birds commute between their breeding areas in Central Europe and the warmer southern Europe or North Africa , where they stay in winter (short-range migrants).
- Others switch between Europe and Central Africa (medium-distance migrants) or South Africa (long-distance migrants).
- Some bird species migrate from Scandinavia or Siberia to Central Europe (often medium-distance migrants).
- Many arctic waterfowl overwinter on the Lower Rhine and on the North Sea coast.
- Many songbirds from Northern and Eastern Europe seek warmth in Western and Central Europe in winter.
As reported in September 2007, a female has bar-tailed godwit called E7 carried out a 11,500 km long flight (with no height calculation) from Alaska to New Zealand nonstop. Like several others, the bird was equipped with a transmitter. As far as is known, this animal holds the flight distance record for migratory birds.
According to Peter Berthold , most birds migrate both in northern Germany and in the Swiss Central Plateau at altitudes below 1000 meters. However, swans have also been observed in Europe flying at 8000 to 8500 meters. Even the Himalayas are crossed by many species of migratory birds, with the animals reaching heights of 7000 to 10000 meters.
Night and day flights
Most of the bird migration happens at night. With the help of radar positioning, the behavior of migratory birds during the Trans- Saharan migration in the Mauritania area could be described. The by Schmaljohann et al. (2007) observed birds migrating south from Europe in autumn and north again in spring, mostly on the ground during the day and mostly migrating at night. The solitary birds rose at sunset and landed in the sand as soon as the sun rose. Previously, some researchers had assumed that they would fly over the hot desert areas of the Sahara in a 40-hour non-stop flight.
Observations by the working group of the Swiss Ornithological Institute in Sempach indicate that it is more energy-saving , especially for lightweights such as Fitis , pied flycatchers and garden warblers, if they spend the hot hours resting on the ground and do not fly in the turbulent air masses.
Large and heavy birds prefer daytime and overland flight and mostly use the V-formation to save energy. You can be carried upwards by the heated air masses and then sail in the desired direction of pull (see thermal sailors ).
More flight strategies
Most species of birds migrate in Breitfrontzug , that is a wide area, as long as no barriers pretend (like mountains or oceans) route. If migratory birds are tied to certain resting places on their way, they move on migratory roads (narrow front migration).
For some species and populations, the way there and back are different, see loop train .
The causes of bird migration
The most important ecological cause of bird migration is the seasonally extremely different food supply in the breeding areas: While insectivores find plenty of food around the Baltic Sea in spring and summer, for example, it is so cold there in winter that hardly any insects fly around and large bird populations therefore lack food would suffer and perish. Conversely, so many birds gather in the winter quarters further south that there is too little food to lay eggs and the young birds can later be supplied with food.
Moving to the north in summer also has the advantage that the very long daylight lengthens the time for foraging and can thus facilitate the rearing of the young.
The exhausting bird migration is to a certain extent an "emergency solution" (more precisely: an evolutionary adaptation effort ) of those bird species that can basically only survive in a relatively warm climate, but have found a way out in the course of tribal history to be able to colonize comparatively inhospitable areas.
Whether a bird draws wherever he moves, and when with him the migratory restlessness sets in, is genetically determined: Both the flight direction and flight duration are innate. This has, among others, Peter Berthold , Eberhard Gwinner and Wolfgang Wiltschko demonstrated experimentally. There are bird species in which sub-populations come from the north in a south-easterly direction around the Alps and other sub-populations in a south-westerly direction. If individuals of both subpopulations are mated with one another, the offspring choose a middle route - in individual cases, curiously, even instead of south to north, towards the British Isles . The mating of long distance migrants with short distance migrants resulted in comparable intermediate behavior in the subsequent generation.
Furthermore, birds were from hatching hand-reared at constant conditions in the laboratory, they never that contact with wild counterparts had and knew no seasons. Nevertheless, they showed the typical for migratory birds migratory restlessness , that is an increase of motor activity in the autumn and spring. However, the distance from one autumn migratory unrest to the next was usually a little less than a year. This means that the willingness to pull is innate, but the optimal departure date is also at least slightly influenced by environmental influences (e.g. weather conditions and the availability of food).
The exact physiological, especially the hormonal mechanisms that ultimately lead to the onset of bird migration are currently the subject of intensive research.
Bird migration and evolution
The way in which bird migration developed in the course of the tribal history of birds is speculative, as there are no fossil records for such behavior. Only the mechanism that maintains the innate ability to migrate can be understood : If the food supply at the destination of the seasonal bird migration is good, most of the migrating birds that have arrived there will survive. On the other hand, if the food supply is insufficient, they die. This means that only those birds that, thanks to their genetic makeup, choose both the right direction and an appropriate flight route, can pass on their genes and thus their migratory behavior to the next generation.
Bird migration is still stabilized today through the selection of the most adapted individuals.
In order to orientate themselves on their migration path, the birds use an " inner compass ", but also astronomical navigation (position of navigation stars or the position of the sun) and landmarks. Usually different information is used at the same time.
Despite the distinctive orientation of the migratory birds, the goal is not always achieved. For example, weather conditions can cause the birds to overshoot the target ( migration prolongation ). If they stray far from their destination or the train route, one speaks of wandering guests .
The “inner compass” is probably the result of a magnetic sense , more precisely: of magnetic field receptors , with the help of which the birds can perceive the angle of inclination of the earth's magnetic field . In robins , this receptor is apparently in the right eye: if you cover the eye, they lose the ability to orientate themselves in the earth's magnetic field. In domestic pigeons it was also shown that there is a second magnetic sensor in the skin of the upper beak ; he could measure the strength of the magnetic field.
According to a study from Oldenburg, electrosmog in the frequency range from two kilohertz to five megahertz affects the magnetic compass of migratory birds. This relatively low frequency range can already be found in normal household appliances. Once the birds have moved away from the electrosmog area, they can easily orientate themselves again to the earth's magnetic field.
Some birds can orientate themselves using the night sky . As early as the 1970s, this was demonstrated in warblers in a planetarium , whereby the overall rotation of the starry sky seems to be taken into account and less specific changes between the stars. Hand-raised indigo finch that were never seen as juveniles the stars were, later on the train unable to orient themselves like their wild counterparts - which can be taken as an indication that the celestial navigation ( "Star Compass") learned must become. If, on the other hand, such hand-raised animals were presented with a starry sky rotating around the north star in a planetarium between the fledglings and the first autumn migration, they showed normal migration behavior towards the south.
Position of the sun
The Sun can some birds, so at sunrise and sunset, use for orientation. Their ability to perceive UV light could make orientation based on the position of the sun easier, even in hazy weather. There are also studies that suggest that at least some species of birds also perceive the polarization patterns of the sky , which - depending on the position of the sun - change during the day. How widespread this ability is and whether it is actually used during bird migration is unclear.
Some research indicates that landmarks are also used for orientation, for example the route of motorways and the lighting of large cities. It can therefore be assumed that light pollution and laser shows at major events can disrupt the orientation of migratory birds.
Bird migration and metabolism
In order to be able to cover large distances without ingesting food, the migratory birds not only activate their fat stores that were created before the bird migration began . They even use the proteins of their internal organs, so that these too are at least partially fed into the metabolism for the purpose of generating energy. During this process, also known as combustion , water is released, which contributes significantly to reducing the uptake of drinking water.
In Grauschnäppern was demonstrated in the 1980s that the duration of their stopovers in oases of the Sahara from the fat reserves depends. Well-fed animals stayed there for a shorter time than less well-fed animals. A number of laboratory studies also produced comparable results: animals fed with little food showed less migratory unrest than those conspecifics that could eat a lot of fat.
Recording of flight routes
In the Wadden Sea National Park, large flocks of migratory birds can be seen resting in the Wadden Sea . The Lower Saxony Wadden Sea National Park Administration organizes “Migratory Bird Days” with an extensive program. A tower with long-range telescopes will be set up on the Jade Bay for bird watching .
Detection with radar
Nocturnal flights are beyond visual observation. The fact that birds occasionally migrate at very great heights and thus above the clouds makes visual recording impossible without technical aids. Radar devices, on the other hand, provide information about the intensity of bird migration, largely regardless of visibility. The German Committee for the Prevention of Bird Strikes in Air Traffic (DAVVL eV) has been using this technology since the 1960s to warn air drivers of increased numbers of birds and the associated risk of bird strikes . Depending on the type of radar, information on bird migration times, intensities, heights and spatial distributions can be read from the radar echoes. They are processed into warning messages, the so-called Birdtam , and can be viewed on the DAVVL website.
Reading out the rings of ringed birds enables conclusions to be drawn about their migratory behavior. For example, the ornithological station in Helgoland (today's headquarters in Wilhelmshaven) as the north-west German ringing center has focused on bird migration research since 1910.
Change in tensile behavior
For some years now, a change in the migratory behavior of many bird populations has been observed: more and more bird species that used to be mandatory migrants are now wintering in Central Europe, for example blackcap and chiffchaff . Even white storks remain in winter increased in Germany or in Switzerland . Some starling populations have even completely reversed their migration direction and are moving northwards: to large cities, where they can find sufficient food even in the cold season.
Some scientists associate this development with the consequences of global warming , but also with the plate-tectonic deflection of the warm Gulf Stream , the cold Humboldt Current and other ocean currents that are related to the food supply. Over a longer period of time, the African - Eurasian train system known to us could disappear.
When birds give up their population area due to lack of food and settle in distant regions, one speaks of invasion birds . Because there are no regular return flights, this phenomenon has nothing to do with bird migration.
Dismigration must also be distinguished from bird migration . These are dispersal migrations, especially of young birds, which serve to expand their habitat. The distances covered are much shorter than with bird migration - often only a few kilometers, at most a few hundred kilometers.
- Arrow storks were important for research into bird migration in the 19th century. When they returned to Europe, they had an arrow in their body that suggested a hunt in Equatorial Africa.
- The hand wing index makes it possible to reconstruct the migration behavior of extinct species.
- The Via Pontica is an old Roman road that coincides with the route of some migratory birds.
- Migratory fish can be seen as an aquatic counterpart to migratory birds, as they change their habitat to spawn.
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- In migratory bird research, chapters have to be rewritten. Thanks to ultra-light technology, so-called geodata loggers, knowledge about migratory birds has to be revised. June 4, 2017, SRF
Protection of migratory birds
- Committee against Bird Murder - Association with a focus on protecting migratory birds
- migration-unlimited.org - Protection of migratory birds in the Mediterranean
- EuroNatur Foundation - program to protect migratory birds in Europe
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Electrosmog disrupts the orientation of migratory birds. In: idw-online.de from May 7, 2014
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- "Migratory Bird Days" website
- Rembert Unterstell: "We'll be gone." In: forschung - The magazine of the German Research Foundation of October 4, 2013, pp. 10-13, doi: 10.1002 / fors.201390043 . Article about the migration of the wheatear birds . Full text