As migration is defined as the annual flight of migratory birds from their breeding grounds to their wintering grounds and back again. Every year, an estimated 50 billion migratory birds traveling, of which about five billion between Europe and Africa around the world.
As a migratory species is referred to when she spends several seasons in different places. Obligate migratory birds always leave about the same time and independently of climatic conditions their breeding grounds, fly to remain approximately the same route to their winter quarters, and return the following spring.
The counterpart to the migratory bird is the state bird. Bird species in which only part of the population moves is called a partial migrant. Another hybrid form of bar- birds: leave their breeding grounds in winter, but stay in the same latitudes.
- 2.1 Ecological causes
- 2.2 Genetic causes
- 2.3 Physiological causes
- 2.4 Bird migration and evolution
- 5.1 Visual observation
- 5.2 detection with radar
- 5.3 Other Methods
Characteristics of bird migration
Distances and migration routes
After the distance traveled to different short-distance migrant, middle -distance migrants and long-distance migrant.
- Many migratory birds commute between Central Europe, where they spend the summer, and the warmer southern Europe and North Africa, where they reside in the winter (short- distance migrants ).
- Other alternate between Europe and Central Africa (Middle -distance migrants ) or South Africa (long- distance migrants ).
- Some bird species migrate from Scandinavia or Siberia to Central Europe (often middle -distance migrants ).
- Many arctic waterfowl wintering in the Lower Rhine and the North Sea coast.
- Many song birds from North and East Europe in the winter, the heat in Western and Central Europe.
As reported in September 2007, a female bar-tailed godwit has performed with the designation E7 a 11,500 km long flight from Alaska to New Zealand nonstop. The bird was like several other equipped with a transmitter. This animal holds so far as is known, the flight distance record for migratory birds.
Peter Berthold According attract the most birds in both North Germany and the Swiss Plateau at altitudes below 1000 meters. However, even swans have been observed in Europe, who flew in 8000-8500 meters. Even the Himalayas is crossed by many migratory bird species, the animals reach heights 7000-10000 meters.
Flights in the night and day
The majority of bird migration occurs at night. With the help of radar fixes the behavior of migratory birds in the trans-Saharan train in the territory of Mauritania could be described. The narrow Johann et al. (2007) observed, in the fall of Europe to the south and again withdrawing to the north in the spring birds keep mostly during the day on the ground and pull predominantly at night. The individually traveling birds rose at sunset in the air and landed in the sand once the sun came up. Previously, some researchers had assumed that they fly over the hot desert regions of the Sahara in a 40-hour non-stop flight.
The observations of the working group of the Swiss Ornithological Institute suggest that it is forces gentler specifically for lightweights like Willow Warbler, Pied Flycatcher and Garden Warbler, if they spend the hot hours resting on the ground and not flying in the turbulent air masses.
Large and heavy birds prefer the flight on the day and over the country to save energy. They can be carried by the heated air mass upwards and then sail to the desired load (see thermal gliders).
Other airlines strategies
Most species of birds migrate in Breitfrontzug, that is a broad area, as long as no barriers ( such as mountains or oceans ) specify the route. If migratory birds are bound on their way to certain rest areas, they feed on flyways ( Schmalfrontzug ).
In some species and populations there and back are different, see Schleifenzug.
The causes of bird migration
The biological bases of bird migration can be discussed from both an environmental from genetic and physiological perspective. In the evolution of bird migration, these aspects have worked together.
Most important environmental cause of bird migration is the seasonal extremely diverse food supply in the breeding areas: While insectivores, for example, within the Baltic abundant find food in the spring and summer, it is there so cold in winter that hardly fly about insects and large bird populations, therefore, lack of food would suffer and perish. Conversely, gather in the more southern wintering grounds so many birds that also there the food is scarce, as that still laid eggs and the young birds could be supplied later with food.
The Dodge north in the summer also has the advantage that the very long Taghelligkeit may extend the time for foraging and thus promote the rearing of the young.
The strenuous migration is so far something of a " last resort " (more precisely, an evolutionary adaptation performance ) of those species of birds that can generally survive only in a relatively warm climate, but have found in the course of evolution a way to colonize relatively inhospitable regions can.
Whether a bird draws, where he pulls out and wondering when he uses the restlessness, is genetically determined: Both the flight direction and flight duration are innate. This has been demonstrated experimentally, among others, Peter Berthold, Eberhard Gwinner and Wolfgang Wiltschko. Thus, there are birds, which fly sub-populations from the north in a southeasterly direction to the Alps around and other sub-populations in a south-westerly direction. If individuals of both subpopulations were mated, select the offspring a middle way - in some cases, curiously, even instead of south to north, in the direction of the British Isles. The mating of remote openers with short- distance migrants gave similar intermediate behavior in the subsequent generation.
Furthermore, birds were hand-reared from hatching to under constant conditions in the laboratory, so they never had contact with wild conspecifics and knew no seasons. Nevertheless, they showed the typical migratory restlessness, that is an increase of motor activity in autumn and spring. However, the distance from an autumn migratory restlessness to the next was usually slightly shorter than a year. This means that the willingness to pull is indeed innate, the optimal departure date is also influenced by environmental factors ( for example, by weather conditions and food availability ), at least to a small extent.
The exact physiological, especially the hormonal mechanisms that ultimately lead to the onset of migration, are currently the subject of intensive research.
Bird Migration and Evolution
In what ways do the migration has emerged in the course of phylogeny of birds, is speculative, since there is no fossil traditions for such behaviors. Only the mechanism is understandable that causes the receipt of the innate ability to draw: Is the food supply at the destination of the seasonal migration season well, then survive there most of the incoming migratory birds. Is the food supply, however, insufficient, they die. This means that only those birds, thanks to their genetic choose both the right direction as well as an appropriate route can pass on their genes and thus their migratory behavior to the next generation.
Bird migration is therefore still stabilized by the selection of the best adapted individuals.
In order to orient themselves on their migration route, the birds use an "inner compass ", but also the position of the stars or the sun and landmarks.
- The "inner compass " is - it at least suggest some recent studies suggest - probably the result of magnetic receptors in the eye, by which the birds can perceive the angle of inclination of the geomagnetic field. In robins, this receptor is obviously in the right eye: Covers one eye from, they lose the ability for orientation in Earth's magnetic field. In pigeons (not only in pigeons ) has been described recently in a number of scientific publications, that a magnetic sensor located on the upper part of the beak; he could measure the strength of the magnetic field. The study of the magnetic sense currently infected but yet (mainly due to lack of funds to conduct basic biological research ) in its infancy; therefore safe, that is internationally recognized, reproducible findings are hardly.
- Birds can orient themselves according to the starry sky and enforcement. Already in the 1970s it has been demonstrated among others warblers in a planetarium, where especially the overall rotation of the night sky seems to be considered less and certain changes of the stars to each other. Hand -reared Indigo finches that were never seen as hatchlings the stars were, later on the train will not be able to look like their wild conspecifics to orient - what can be regarded as evidence that the "star compass " must be learned. If such hand-reared animals, however, in a planetarium between fledging and the first autumn migration presented a rotating around the North Star starry sky, they showed a normal southward directed migratory behavior.
- The sun, the birds, thanks to their ability to perceive ultraviolet light, even on cloudy days recognize. There are also some studies that suggest that at least some species of birds also perceive the polarization pattern in the sky, which - change throughout the day - depending on the position of the sun above the horizon. The polarization is at its largest when the light is scattered at an angle of 90 ° relative to the sun. How widespread is this ability and whether it will actually be used during migration, but is still largely unknown.
- Some research finally suggest that the orientation landmarks are used, for example, the course of highways and the lights of big cities. It is therefore likely that the light pollution and laser shows can disturb the orientation of migratory birds during major events.
Despite these abilities of migratory birds, the goal is not always achieved. For example, can cause weathering that the birds over the top shoot ( Zugprolongation ). If they stray far from their destination, or the migration route, it is called wanderers.
Migration and metabolism
To be able to cover large distances without food intake, activate the migratory birds not only its scale before the start of migration fat stores. They even resort to the proteins of their internal organs, so that they are at least partially supplied to the metabolism in order to gain energy. This also called combustion process of fat and protein water is released, which contributes to a considerable extent, to reduce the intake of drinking water.
In Grauschnäppern was demonstrated in the 1980s that the duration of their intermediate stations depends on the fat reserves in the oases of the Sahara. Well -fed animals stayed there on less than less well-fed. Comparable results also brought a number of laboratory studies: With little food supplied animals showed less restlessness than those fellows who could erode abundant fat.
Detection of the flight paths
In the Wadden Sea National Park large flocks of migratory birds on the catch in the Wadden Sea can be observed. The National Park Administration Lower Saxon Wadden Sea performs " migratory bird days " through an extensive program. At the Jade Bay tower is situated to bird watching, standing on the far-reaching telescopes available.
Detection with radar
Nocturnal flights go beyond the visual observation. Also the fact that birds occasionally pull at very high altitudes and thus above the clouds, makes a visual recording without technical aids impossible. Radars, however, give largely independent of visibility information about the intensity of bird migration. The German Committee for the Prevention of bird strikes in aviation ( DAVVL eV) since the 1960s makes this technique already of alerting Airmen from increased occurrence of birds and associated bird strike advantage. Depending on the type of radar is information intensities read about bird times, heights and spatial distributions of the radar echoes. You are on the alerts, the so-called BIRDTAM, processed and can be viewed on the website of DAVVL.
The readout of the rings of ringed birds allows conclusions to their migratory behavior. For example, the Ornithological Helgoland busy ( today headquartered in Wilhelmshaven ) as northwestern German ringing since 1910 mainly with the bird migration research.
For a more precise detection of routes are larger birds with satellite transmitters, small provided with Helldunkelgeolokatoren.
Change in migratory behavior
For several years there has been a change in the migratory behavior of many bird populations find that more and more species of birds that were mandatory puller earlier, hibernate now in Central Europe, for example, Blackcap and Chiffchaff. Some Star populations have their direction of pull even fully reversed and move to northern directions: in large cities, where they find an adequate food supply during the cold season.
Some scholars take this development with the consequences of global warming in context, but also with the plate tectonic deflection of the Gulf Stream, the cold Humboldt current and other ocean currents, which related the food supply. Over a longer period of time could we know the African- Eurasian train system disappear.
If birds because of lack of food to give up their territory and population to settle in distant regions, it is called invasion birds. Because it is not regular round-trip flights, this phenomenon has nothing to do with the migration.
Furthermore, to distinguish it from bird migration is the Dismigration. These are distraction hikes in particular of young birds, which serve to increase the habitat. The distances involved are much shorter than in the bird migration - often only a few kilometers, at most a few hundred kilometers.