Migratory birds find their way with astonishing accuracy. But how they determine their east-west position has long been unclear. Now Oldenburg migratory bird expert Henrik Mouritsen and an international team of scientists have shown that reed warblers (Acrocephalus scirpaceus) determine their east-west position by recognising the angle at which the magnetic North Pole deviates from the geographic North Pole.
Determining longitude was a major challenge for sailors for centuries. It was not until the 18th century, thanks to the inventions of clockmaker John Harrison, that people were able to determine their east-west position using two very accurate clocks - one on local time and one on home time. However, migratory birds find their way without having two clocks. Prof Dr Henrik Mouritsen from the University of Oldenburg and an international team of scientists have now demonstrated this by studying reed warblers (Acrocephalus scirpaceus): The migratory birds determine their east-west position by perceiving the angle at which the magnetic north pole deviates from the geographic north pole. The researchers present their findings in the current online first issue of the renowned journal Current Biology.
Migratory birds navigate with astonishing precision on their flights, which are often several thousand kilometres long. They orientate themselves with the help of the position of the sun, the stars and the Earth's magnetic field. So far, scientists have been able to explain how birds can determine their compass direction and their north-south position. But how they determine their east-west position has been one of the greatest mysteries of migratory bird research for decades. The research team from Oldenburg, Rybachy and St. Petersburg (both in Russia) and Bangor (Wales) has now shown that adult reed warblers determine what is known as magnetic declination. This means that the birds perceive the angle at which the magnetic North Pole deviates from the geographic North Pole. "This is the first time we have provided an answer to the question of how birds determine their east-west position," says Mouritsen.
In an earlier study, the researchers had already discovered that reed warblers cannot determine their longitude position by time shifts like humans. In further studies, the researchers were also able to show that experienced birds can compensate for this if they are actually moved from east to west and that the Earth's magnetic field must play a decisive role in this. However, it remained unclear exactly which parameter helps the animals. The researchers therefore hypothesised: Perhaps the birds can determine the angle between the magnetic north pole and the geographic north pole using their star and magnetic compasses. This declination angle changes very regularly from east to west in Europe.
The scientists tested this hypothesis with an experiment: they temporarily placed 15 adult reed warblers in cages during the autumn migration in Rybachy, Russia, and tested them in so-called orientation funnels. These were equipped with coil systems that can simulate a uniform magnetic field. For the experiment, the scientists rotated the magnetic field by 8.5 degrees compared to the natural magnetic field on site. The angle of deviation now corresponded to that of the magnetic field in southern Scotland, 1,200 kilometres away. All other potential cues for the birds, such as the strength and inclination of the magnetic field as well as odours and visual impressions, remained the same. This revealed "an astonishing effect", reports Mouritsen: "The birds changed their orientation by 151 degrees from west-south-west to east-south-east, thus compensating for the virtual displacement." In the same experiment with young reed warblers, however, the birds reacted disorientated. The latter confirms what researchers have known for some time: The birds first have to learn to orientate themselves using a magnetic map.
According to Mouritsen, the results of the study shed a completely new light on researchers' ideas about how birds navigate. One of the cornerstones of orientation research to date has been that birds have completely separate systems with which they determine their location (map) and the direction of flight (compass). "However, since birds can apparently use two compasses to determine their east-west position, this strict separation between map and compass is no longer correct," says the neurobiologist. In his opinion, it is therefore not only necessary to look at future experiments with different eyes, but also to draw completely different conclusions from earlier experiments on bird migration.
