Abstract
The magnetic compass is an important element of the avian navigation system, which allows migratory birds to solve complex tasks of moving between distant breeding and wintering locations. The photochemical magnetoreception in the eye is believed to be the primary biophysical mechanism behind the magnetic sense of birds. It was shown previously that birds were disoriented in presence of weak oscillating magnetic fields (OMF) with frequencies in the megahertz range. The OMF effect was considered to be a fingerprint of the photochemical magnetoreception in the eye. In this work, we used miniaturized portable magnetic coils attached to the bird’s head to specifically target the compass receptor. We performed behavioural experiments on orientation of long-distance migrants, garden warblers (Sylvia borin), in round arenas. The OMF with the amplitude of about 5 nT was applied locally to the birds’ eyes. Surprisingly, the birds were not disoriented and showed the seasonally appropriate migratory direction. On the contrary, the same birds placed in a homogeneous 5 nT OMF generated by large stationary coils showed clear disorientation. On the basis of these findings, we suggest that the disruption of magnetic orientation of birds by oscillating magnetic fields is not related to photochemical magnetoreceptors in their eyes.
Highlights
The magnetic compass is an important element of the avian navigation system, which allows migratory birds to solve complex tasks of moving between distant breeding and wintering locations
The radical pair model predicts that oscillating electromagnetic field in the lower megahertz range (1–100 MHz) can disrupt the magnetic compass due to the electron paramagnetic resonance effect[19]
The mean direction of birds obtained indoors was similar to the mean autumn migratory direction of the same species, according to recoveries of birds ringed on the Courish Spit (α = 213°, n = 14, r = 0.96, p «0.001, 95% CI = 205°–222° 37; and unpublished data of the Biological Station Rybachy) and to the data obtained in previous experiments in Emlen funnels in garden warblers (α = 194°, n = 38, r = 0.41, p = 0.001, 95% CI = 169°–229° 24; these two distributions do not differ according to MWW test: W = 0.3, p = 0.86)
Summary
The magnetic compass is an important element of the avian navigation system, which allows migratory birds to solve complex tasks of moving between distant breeding and wintering locations. As applied to the magnetic compass of birds, the model of spin-dependent chemical reactions, known as the radical pair model (RPM), is the most popular one[14,15,16,17,18] According to this model, the primary biophysical detection of the magnetic field is provided by molecules of a photosensitive protein, cryptochrome, located in the retina of the bird’s eye. Disruption of the magnetic compass in presence of oscillating magnetic field has been suggested as a diagnostic tool for the radical pair reaction mechanism underlying the magnetic compass[15] This effect was experimentally observed in dozens of experiments performed by at least three groups independently in birds[20,21,22,23,24] and in mammals[25]; this would seem sufficient to confirm RPM beyond reasonable doubt. In order to shed light on this controversial issue, which is extremely important for understanding the magnetic orientation of birds, we conceived an experiment aimed at spatial localization of the compass receptor via its sensitivity to oscillating magnetic fields
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