Avian magnetic compass can be tuned to anomalously low magnetic intensities - PubMed (original) (raw)

Avian magnetic compass can be tuned to anomalously low magnetic intensities

Michael Winklhofer et al. Proc Biol Sci. 2013.

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.

Keywords: ability to adjust; functional window; magnetic compass; radical-pair mechanism; radiofrequency fields.

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Figures

Figure 1.

Figure 1.

Orientation of European robins during spring migration in the local geomagnetic field (control, C) and, after pre-exposure, in the 4 µT field (4 µT), in this field with a radiofrequency field of 1.315 MHz, 480 nT added (4 µTRF), and with the skin of the upper beak locally anaesthetized by Xylocaine (4 µTXy). The triangles at the periphery of the circle mark the mean headings of the individual birds, the arrows represent the grand mean vector in relation to the radius of the circle = 1, and the two inner circles are the 5% (dashed) and the 1% border of the Rayleigh test [22].

Figure 2.

Figure 2.

Orientation of robins in autumn in the local geomagnetic field (control, C) and, after pre-exposure, in the 4 µT field (4 µT). Symbols as in figure 1.

Figure 3.

Figure 3.

Orientation of robins in autumn tested in the 4 μT field after 8 h pre-exposure in the 4 μT field before each test. In the four diagrams, the triangles at the periphery of the circle indicate the individual headings of the 16 test birds during the 1st, 2nd, 3rd and 4th test. Symbols as in figure 1.

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