They are traveling with a “sixth sense” – migratory birds are guided on their journeys by the earth’s magnetic field. But how do they know when to end their flight to land at their traditional breeding grounds? According to a study, they are based on the local characteristics of inclination – the angle of inclination between field lines and the earth’s surface of the earth’s magnetic field.
Every year in autumn they embark on mighty intercontinental flights, only to return to their respective breeding grounds in the spring from their “winter vacation” in the south. The amazing orientation skills of the migratory birds during these migrations have fascinated scientists for a long time. It is attributed to the ability of so-called magnetoreception: some animals can be guided by the magnetic field of our planet. There is evidence that the birds’ internal compass is in the eye. According to this, iron oxide particles in certain cells indicate characteristics of the earth’s magnetic field lines. Apparently, the animals can use this sixth sense to adjust their flight direction.
However, it is still unclear how migratory birds recognize where they have to end their journey in order to land at their traditional destinations. “Studies are making it increasingly clear that bird migration follows a fixed program that birds inherit from their parents. But how they return to the same place year after year with great precision remains a mystery,” says Joe Wynn from the Institute for Bird Research in Wilhelmshaven. To address this question, Wynn and his colleagues targeted the reed warbler (Acrocephalus scirpaceus) as a model. These small songbirds spend the summer in their respective breeding grounds in many regions of Europe – while their winter quarters are in sub-Saharan Africa.
stop signals on the lane
The reed warbler has long been the focus of ornithologists: specimens have been individually marked with small metal foot rings for almost a hundred years. Both the ringing sites and the discovery sites are recorded centrally for the whole of Europe. “This data is a fantastic tool for answering questions about bird migration because it was collected over a very large area over so many years,” says Wynn. As part of their study, he and his colleagues analyzed ringing data from almost 18,000 reed warblers between 1940 and 2018. The team also recorded the characteristics of the earth’s magnetic field at the sites. Statistical methods were used for the evaluation in order to uncover possible patterns that contain clues to the orientation system of the birds.
As the researchers report, their results showed that the birds use certain geomagnetic coordinates as stop signals for their migrations. They are therefore based on the local characteristics of the so-called magnetic inclination. This term describes the angle of inclination between the magnetic field lines and the earth’s surface. Once the inclination reaches a certain value, the birds stop moving, the results suggest. According to the authors, the birds apparently learn the angle of inclination before they set off and then use it to return.
When the sixth sense says “stop”.
Interestingly, characteristic slight inaccuracies in the landing sites of the birds provided the scientists with the clues: They established a clear connection between the sites and the slow drift of the earth’s magnetic field. The researchers explain that its field lines can shift by a few kilometers in different directions from year to year due to movements of the liquid iron in the Earth’s core. “However, the magnetic field does not change very much at a specific location. It therefore makes sense for the birds to choose a specific magnetic field value as the destination of their journey,” explains Wynn.
According to the scientists, the signaling function of inclination is the best way for birds to return to their respective breeding sites because it is the most reliable component of the Earth’s magnetic field. However, they found no evidence for the role of other components, such as the field strength or the deviation between magnetic and geographic north. However, they warn that their results and conclusions should now be verified in other bird species.
In conclusion, Henrik Mouritsen from the University of Oldenburg says: “The ability of birds weighing less than a teaspoon to precisely locate their breeding site after traveling halfway around the world is perhaps one of the most amazing aspects of bird migration”. As he points out, the new insights into this ability are based in large part on the engagement of amateur birders. The team also hopes that their study can inspire even more people to observe birds and get excited about researching their amazing abilities.
Source: University of Oldenburg, specialist article: Science, doi: 10.1126/science.abj4210