During migration, birds navigate using both a genetically inherited sense of direction and a magnetic sense that enables them to use the Earth’s magnetic field lines to work out their location. It was previously unclear whether these migratory birds also used an experience-based “map” to find the way to their destination. Scientists at the Max Planck Institute for Ornithology in Radolfzell exposed robins (Erithacus rubecula) on a migration stopover to a strong magnetic pulse, temporarily disrupting their magnetic sense. As a result, the orientation of birds with prior migratory experience became less precise.
By contrast, the precision of departure direction in juvenile birds, which were migrating for the first time, was not affected by the pulse treatment. The migration-inexperienced juvenile birds had apparently not yet created a magnetic map. It follows, therefore, that this magnetic map sense is based on experience and is disrupted by such magnetic pulses.
In spring and fall, thousands of birds migrate to their breeding and wintering grounds. The overall bearings for this migration are genetically inherited. However, birds also possess an additional magnetic sense that allows them to navigate based on cues from Earth’s magnetic field. The strength of this magnetic field varies from the poles to the equator, thus providing information as to where exactly a bird is at a specific moment in time. Researchers believe that the birds use this magnetic sense to build up “maps” during migration, which they then use to help them navigate in the following years.
The scientists at the Max Planck Institute for Ornithology have now investigated what effect a magnetic pulse has on the orientation ability of wild robins. They attached minute radio transmitters to the birds at a stopover during their migratory flight, enabling them to determine the departure direction of the robins after the pulse treatment. The researchers made a distinction between older birds that had already migrated at least once, and juvenile birds on their first migratory journey.
They discovered that the older birds which had received magnetic pulse treatment flew off in the wrong direction far more often than the birds in the control group that had not been exposed to the pulse. The effect was strongest in the birds that departed within ten days of treatment. “The pulse must have reset the robins’ magnetic maps,” says Richard Holland of the Max Planck Institute in Radolfzell. “Thus they initially flew in the wrong direction.”
This would also explain why the departure orientation of juvenile birds that had received the same treatment was unaffected. “The juvenile birds had never migrated before and so had not yet created a magnetic map that we could reset,” explains Holland. Thus it follows that the magnetic sense of robins is significantly influenced by their prior migratory experience.
Richard Holland and his team will continue to explore the mechanisms of migration. “We still don’t know for sure where the receptor for this magnetic information is actually located,” he explains. It has been proposed that the birds have ferrimagnetic particles in their beaks; however, alternative theories suggest systems in the eye, or in the balance apparatus of the ear.
Source: Max Planck Institute