Reed warblers are amazing birds: although they are not even as long as a hand and weigh less than a teaspoon, they migrate more than 6,000 kilometers from their breeding grounds in Europe to their winter quarters south of the Sahara in late summer and autumn:
“The following spring they move again – and they return to almost the same place they left the previous year.” Joe Wynn from the Institute for Bird Research in Wilhelmshaven is investigating how the small birds do this, together with colleagues from the University of Oxford .
Reed Warbler perfect for study
The team chose the reed warblers for one reason in particular: “They are perfect for our studies because they are very often ringed. If a bird is later recaptured, you can identify it and infer all sorts of cool things: about their demographics, their ages, but also their migrations.”
Ringing sites and find sites are recorded centrally for the whole of Europe, so that the data from around 18,000 birds was included in the study. Reed warblers migrate at night. And because very few of them live to be two years old, the route to their winter quarters must be genetically programmed.
Amazing precision on return
But the place they return to is different, says Joe Wynn: “They find their way back to the place where they hatched with such amazing precision that they must have learned more information in order to then find them on their way home.”
The suspicion: They use the earth’s magnetic field. How birds perceive this magnetic field is currently the subject of intensive research. The ornithologists follow the theory that birds register magnetic field lines with the help of special molecules in their eyes – and that the animals use this ability to determine their flight direction and also their location. So the researchers analyzed the geomagnetic field data in parallel to the ringing data.
Angle of inclination of the earth’s magnetic field
“There are several parameters that we think birds can pay attention to. One is the inclination, the angle between the magnetic field lines and the earth’s surface. They are almost parallel at the equator and perpendicular at the poles. Another factor is the strength of the Earth’s magnetic field. Finally, there is the angle between the magnetic and true north poles. If the birds can remember such cues, it should point them home.”
However, the earth’s magnetic field is not static, but shifts by a few kilometers from year to year. The researchers therefore investigated whether the birds’ breeding grounds also shifted with the magnetic field. And indeed: “The birds seem to be sensitive to the inclination of the magnetic field, i.e. to the angle between the magnetic field lines and the earth’s surface. They shifted their breeding grounds accordingly,” explains ornithologist Joe Wynn.
Breeding place to predict almost exactly
“Surprisingly, we found no evidence that they react to the other two factors.” To be more precise: When the researchers included the angle between field lines and the earth’s surface in their analysis, they were able to predict the breeding site with a statistical deviation of just over one kilometer For the other two parameters, the deviations were significantly larger at 20 to 200 kilometers.
So the reed warblers seem to use the magnetic inclination like a stop sign: as soon as this angle reaches a certain value, they stop flying and look for a nesting site. However, the researcher emphasizes that their findings are not proof. This requires experiments and analyzes with other migratory birds.