Disruption of magnetic orientation in migratory songbirds by radiofrequency magnetic fields is mediated by a specialized sensory system

Direction finding in migratory birds is known to be affected by oscillating magnetic fields (OMFs) in the radiofrequency range. This experimental fact was earlier interpreted in terms of the direct influence of OMFs on electron spins in the molecule of cryptochrome that, according to the photochemical model of magnetoreception, serves as the primary sensor of the geomagnetic field. In our experiments, birds (pied flycatchers) were subjected to OMFs with carrier frequencies of 1.41 and 1.5 MHz with the square wave amplitude modulation at 500 Hz. The modulated OMF, having twice less mean power than that without modulation, nevertheless caused disorientation at a lower amplitude of the carrier wave than the unmodulated OMF. Since the prediction of the photochemical theory is exactly the opposite, we conclude that the effect of OMFs on the magnetic orientation of birds is not related to the decoherence of electron spins in cryptochrome. Instead, we suggest that OMFs are perceived by a separate sensory system, most likely based on electromagnetic induction. This hypothetic sensory system, specialized at the detection of magnetic perturbations caused by solar flares or thunderstorms, might react to shape rather than to the mean power of the detected signal, which would explain its higher sensitivity to modulated OMFs.