title: Avian magnetic compass can be tuned to anomalously low magnetic intensities.
abstract: The avian magnetic compass works in a fairly narrow functional window around the intensity of the local geomagnetic field, but adjusts to intensities outside this range when birds experience these new intensities for a certain time. In the past, the geomagnetic field has often been much weaker than at present. To find out whether birds can obtain directional information from a weak magnetic field, we studied spontaneous orientation preferences of migratory robins in a 4 µT field (i.e. a field of less than 10 per cent of the local intensity of 47 µT). Birds can adjust to this low intensity: they turned out to be disoriented under 4 µT after a pre-exposure time of 8 h to 4 µT, but were able to orient in this field after a total exposure time of 17 h. This demonstrates a considerable plasticity of the avian magnetic compass. Orientation in the 4 µT field was not affected by local anaesthesia of the upper beak, but was disrupted by a radiofrequency magnetic field of 1.315 MHz, 480 nT, suggesting that a radical-pair mechanism still provides the directional information in the low magnetic field. This is in agreement with the idea that the avian magnetic compass may have developed already in the Mesozoic in the common ancestor of modern birds.
authors.affiliation: Department für Geo- und Umweltwissenschaften, Ludwig-Maximillians-Universität München, Theresienstrasse 41/IV, D-80333 Munich, Germany.
authors.forename: Michael
authors.initials: M
authors.lastname: Winklhofer
authors.forename: Evelyn
authors.initials: E
authors.lastname: Dylda
authors.forename: Peter
authors.initials: P
authors.lastname: Thalau
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Wiltschko
authors.forename: Roswitha
authors.initials: R
authors.lastname: Wiltschko
mesh_terms: D025041:Animal Migration; D000818:Animals; D001502:Beak / Q000502:physiology / Q000528:radiation effects; D001717:Birds; D005426:Flight, Animal / Q000502:physiology*; D060526:Magnetic Fields; D060328:Magnetic Phenomena; D009949:Orientation / Q000502:physiology* / Q000528:radiation effects; D011846:Radio Waves; D020308:Songbirds / Q000502:physiology*
keywords: ability to adjust; functional window; magnetic compass; radical-pair mechanism; radiofrequency fields