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. 2017 Aug;14(133):20170364.
doi: 10.1098/rsif.2017.0364.

Very Weak Oscillating Magnetic Field Disrupts the Magnetic Compass of Songbird Migrants

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Free PMC article

Very Weak Oscillating Magnetic Field Disrupts the Magnetic Compass of Songbird Migrants

Alexander Pakhomov et al. J R Soc Interface. .
Free PMC article

Abstract

Previously, it has been shown that long-distance migrants, garden warblers (Sylvia borin), were disoriented in the presence of narrow-band oscillating magnetic field (1.403 MHz OMF, 190 nT) during autumn migration. This agrees with the data of previous experiments with European robins (Erithacus rubecula). In this study, we report the results of experiments with garden warblers tested under a 1.403 MHz OMF with various amplitudes (∼0.4, 1, ∼2.4, 7 and 20 nT). We found that the ability of garden warblers to orient in round arenas using the magnetic compass could be disrupted by a very weak oscillating field, such as an approximate 2.4, 7 and 20 nT OMF, but not by an OMF with an approximate 0.4 nT amplitude. The results of the present study indicate that the sensitivity threshold of the magnetic compass to the OMF lies around 2-3 nT, while in experiments with European robins the birds were disoriented in a 15 nT OMF but could choose the appropriate migratory direction when a 5 nT OMF was added to the stationary magnetic field. The radical-pair model, one of the mainstream theories of avian magnetoreception, cannot explain the sensitivity to such a low-intensity OMF, and therefore, it needs further refinement.

Keywords: bird migration; garden warbler; magnetic compass; orientation; radical-pair model; radiofrequency field.

Conflict of interest statement

We declare we have no competing interests.

Figures

Figure 1.
Figure 1.
Average amplitude of the magnetic field noise measured at the experimental site as a function of the central position of the 100 kHz detection window. Red curve is the signal measured using a single-loop non-resonant antenna. Blue curve is the signal measured using a two-turn loop resonant antenna. Top x-axis shows a low-frequency border of AM broadcast bands (LW, long-wave; MW, middle-wave, 120 m–31 m; SW, short-wave bands).
Figure 2.
Figure 2.
An example of the frequency spectrum of the oscillating magnetic field, measured at an OMF amplitude of 0.6 nT with a resolution bandwidth of 10 kHz.
Figure 3.
Figure 3.
The dependence of the magnitude of the orientation vector r on the amplitude of the oscillating magnetic field (from all our tests).
Figure 4.
Figure 4.
Orientation of garden warblers in the local geomagnetic field (NMF) during the first phase of experiments, autumn 2014–2015: (a) autumn 2014 (α = 184°, n = 22, r = 0.39, p = 0.033; 95% CI = 143°–226°), (b) autumn 2015 (α = 205°, n = 16, r = 0.45, p = 0.036; 95% CI = 164°–247°) and (c) 2 years pooled (α = 194°, n = 38, r = 0.41, p = 0.001; 95% CI 164°–224°). Here α is the direction of the mean vector, r is the length of the mean vector (r varies from 0 to 1), n is the number of birds, p is the significance level and 95% CI is 95% confidence intervals. Dots denote individual directions of each bird; arrow denotes the mean group vector; inner and outer dashed circles are 1 and 5% significance levels of the Rayleigh test, respectively. Radial lines indicate 95% CI. The red triangle and letters mN are the position of the magnetic north.
Figure 5.
Figure 5.
Orientation of garden warblers when 1.403 MHz OMFs with various amplitudes were added to the local geomagnetic field, autumn 2014–2015. (b,d,f,h) Orientation of the experimental birds (subsample) in the OMF during the second phase. (a,c,e,g) Orientation of the same birds (the same subsample) in the NMF during the first phase of experiments. Row (a,b): 20 nT RF field subsample in the NMF ((a) α = 180°, n= 13, r = 0.38, p = 0.16) and OMF ((b) α = 129°, n = 13, r = 0.03, p = 0.99). Row (c,d): 7 nT RF field subsample in the NMF ((c) α = 185°, n = 31, r = 0.45, p = 0.001, 95% CI = 156°–215°) and OMF ((d) α = 201°, n = 31, r = 0.16, p = 0.46). Row (e,f): 1 nT RF field subsample in the NMF ((e) α = 206°, n = 16, r = 0.45, p = 0.036, 95% CI = 164°–247°) and OMF ((f) α = 71°, n = 16, r = 0.29, p = 0.27). Row (g,h): 0 nT RF field subsample in the NMF during the first phase of experiments ((g) α = 198°, n = 15, r = 0.49, p = 0.024, 95% CI = 157°–239°) and the NMF during the second phase when RF coils were turned off ((h) α = 94°, n = 15, r = 0.52, p = 0.016, 95% CI = 55°–132°). For a description of the circular diagrams, see the legend to figure 1.
Figure 6.
Figure 6.
Orientation of garden warblers in three experimental conditions during 2016 autumn experiments: in the NMF ((a) α = 241°, n = 25, r = 0.125, p = 0.68); in an approximate 2.4 nT RF field ((b) α = 139°, n = 21, r = 0.201, p = 0.44); in an approximate 0.4 nT RF field ((c) α = 183°, n = 26, r = 0.349, p < 0.05, 95% CI = 140°–226°). For a description of the circular diagrams, see the legend to figure 1.
Figure 7.
Figure 7.
The amplitude attenuation factor for the vertical OMF as a function of oscillation frequency.

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