Why do birds migrate at certain times of the year

Knowledge : Long haul master

Mountaineers are amazed when they see barge geese flying over rocks and ice on an eight-thousander in the Himalayas while resting on the summit. Humans don't survive long in the thin air up there without additional oxygen. The birds, which weigh almost three kilograms, fly apparently effortlessly over the highest mountain on earth, Mount Everest, on their way from their breeding grounds on the plateaus of Central Asia to their wintering area in India. A flight at an altitude of 9,000 meters, in which each breath pumps only a third of the oxygen into the lungs compared to sea level, is a strenuous performance.

Researchers now know that even if there are some eyewitnesses to this fantastic flight performance, this route is not the rule, but a very rare exception. “Barge geese almost always cross the Himalayas over the lowest passes,” reports Martin Wikelski from the Max Planck Institute for Ornithology in Radolfzell. So the birds take the most comfortable route, still more than 5500 meters above sea level. Wikelski and his colleagues were only able to unmask the modern myth when they pinned small transmitters to the striped geese, which continuously informed them of their whereabouts. “The same applies to almost all animal migrations,” explains the biologist. "There are indeed many theories that researchers were barely able to verify until a few years ago because the technical possibilities were lacking."

Thousands of years ago people observed that swallows gather in large groups in Central Europe in August. A few days later the birds were gone. In autumn, large flocks of geese and cranes migrated over the Baltic coast. The Maasai also observed huge herds of wildebeest and zebras on their migrations in the savannahs of Africa. But because no one knew why these animals set out and where they were going, myths arose that were supposed to explain the coming and going of animals in different seasons. Many researchers can only verify today.

Wikelski is a specialist in animal walks. With the most modern mini-transmitters, he hardly gets to the bottom of substantiated considerations about hiking trails and flight performance. Electrical engineers often develop tiny devices specifically for new experiments that the researcher has come up with. For example with bumblebees. In order to follow their migrations, Wikelski equipped the insects with a super-mini transmitter weighing just 0.2 grams. A bumblebee equipped in this way hummed a few kilometers to the most delicious nectar springs during the fruit blossom at Lake Constance in 2010. And the biologist had discovered another animal migration. Nobody had previously suspected that such small creatures would travel so far.

In general, animals seem to be as restless as humans. Salmon, eels, sharks, whales, many other mammals and even bats, but also birds, insects, crabs and reptiles set off in certain times of the year or maybe only once in their lives. And for different reasons.

One of the most common is finding food. Wikelski recently equipped the giant tortoises on the Galapagos Islands of Santa Cruz and Isabela with transmitters: "The animals cover distances of ten kilometers and overcome differences in altitude of more than a thousand meters on the slopes of the volcanoes," summarizes the researcher. Since there are rainy seasons in the lower regions of the Galapagos Islands, when the vegetation sprouts lush and the turtles migrate towards the coast exactly then, a connection with food is obvious. The researchers still have to investigate which food the animals are actually looking for.

The current world record holders in long-haul flights are also following the food. Arctic terns breed in summer almost everywhere in the northern hemisphere, from the Baltic Sea coast up to the north of Greenland, but winter far in the south on the islands in the Southern Ocean. Ornithologists conclude this from birds that they put small rings of metal or plastic around their legs. So if an arctic tern is observed breeding on Greenland and is ringed there, but later reappears off Antarctica, the animal is evidently commuting between the two regions. In theory, these birds, weighing just one hundred grams, should fly at least 30,000 kilometers each year.

What the birds do between the two areas and which flight routes they choose, the rings rarely reveal. Carsten Egevang from the Greenland Institute of National Resources in Nuuk, the capital of Greenland, and his colleagues therefore equipped eleven arctic terns with loggers weighing 1.4 grams each. These "tachographs" record the length of each day. After returning to the breeding area, the researchers free the birds from the loggers and use the length of the day to determine the animal's whereabouts to an accuracy of around 200 kilometers. In 2010 they published the amazing result in PNAS magazine: The small birds exceed the theoretical flight distance, some arctic terns travel more than 80,000 kilometers a year. The animals skilfully choose their routes in such a way that a tailwind pushes them as often as possible and headwinds only rarely slow them down. They almost always travel over water, for example they fly long distances over the Atlantic and Pacific, thousands of kilometers from the nearest coast.

In addition, with the help of satellite data, the researchers discovered previously unknown resting places for arctic terns. There is always clear water there with lots of small fish swimming in it. Usually the birds fly a few meters above the waves and look out into the sea from there with keen eyes. If they spot a possible prey, they put their wings on a little, tip over and dive through the water surface at a steep angle. The young birds have to learn this technique with great effort, it takes some time before they catch a fish every now and then.

The hunting method only works in broad daylight because the arctic tern needs to see its prey. This also explains the long migrations of the animals. The cold waters in the far north and deep south contain many nutrients and are therefore teeming with fish, but are usually very clear. In summer the nights are short and the arctic terns have enough time for their difficult hunt. In winter, however, there is only a few hours of daylight, further north the sun does not rise at all in the polar night, and the arctic terns would have to go hungry. At the same time, however, the days in the southern summer off the Antarctic are particularly long and the clear waters are teeming with fish. So it is apparently cheaper to dare the long flight to the deep south with its good hunting grounds.

The same applies to the American royal dragonflies on the northeast coast of the USA, which fly thousands of kilometers down to Florida. “They are probably looking for areas where their larvae can find enough food,” suspects Wikelski, who also equipped these animals with mini-transmitters. In the meantime, the biologist at the Sylvenstein reservoir in Bavaria has even equipped the little painted lady with tiny transmitters. These butterflies sometimes even fly over the Alps and are therefore also among the long-distance migrants among animals.

Fluttered animals also migrate long distances, explains Christian Voigt from the Leibniz Institute for Zoo and Wildlife Research in Berlin: "Palm bats live in the tropical rainforest in Africa, but migrate several thousand kilometers into the Sahel zone in the north and as far as Zambia in the south of the continent, when certain fruits ripen en masse there. "

Bats migrate in Europe for completely different reasons. Unlike birds, these mammals can hibernate and thus survive the cold season when there are hardly any insects around to feed on. If the animals overwinter in caves and ruined houses, it is cold there, but severe frost hardly penetrates to them. The situation is different for the sleepers in tree hollows, they could freeze to death in the icy winter nights of Scandinavia or the European northeast. "For this reason, rough-skin bats migrate 2000 kilometers from the Baltic States and Russia to the Lake Constance area or France," suspects IZW researcher Christian Voigt.

Marine biologists suggest a completely different reason for such migrations for humpback whales. Many of these animals live in the cold waters off Alaska. From there, however, females swim more than 5000 kilometers to the coast of Hawaii in just over a month in order to have their calves there. There they find no food and they fast. But the orcas, which are common in cold waters and for which newborn humpback whale calves are a favorite, almost never appear off Hawaii. Some animals don't move, they flee.

Other species also avoid their enemies in this way - but take different risks. A female humpback whale can lose up to a third of her body weight on her way to Hawaii. The monarch butterflies also defy considerable risks when they set off on the five centimeter long wings from their summer areas in North America to the Sierra Nevada in Mexico. They skillfully use the winds often at a height of more than a hundred meters to fly around 50 kilometers south every day. In the evening, the moths gather at rest areas, but in the process they drown by the thousands in bodies of water or are crushed by cars. In Mexico, the survivors sit on sheltered places on the bark of tree trunks so as not to be directly exposed to the cold winter storms.

The cost of hiking, from weight loss to death, should not outweigh the benefits. “When we fitted transmitters to 30 young white storks in Spain, 28 of them died in Africa during their first two years,” explains Wikelski. From dangerous power lines to hunters looking for a Sunday roast, some dangers lurk along the way. In order to estimate such “flight costs”, however, the routes with all the dangers must first be known. However, such cost-benefit analyzes have hardly existed for any animal migration so far. Wikelski believes that future research results will overturn some of today's theory: "Whenever we track animals with transmitters in nature, we receive surprising results that contradict our original ideas."

The migration of birds and the migration of salmon are well known. But with modern technology, researchers are discovering more and more species that are similarly restless. Bats, turtles and dragonflies also migrate.

There are several reasons why animals make long and dangerous journeys: for example, to find food, give birth to offspring or to flee before winter.

Many animal species use landmarks such as railroad tracks or power lines for orientation. However, many birds also use the earth's magnetic field for orientation.

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