Magnetic compass orientation behaviour of Eurasian blackcaps at the predicted 110–120 MHz upper cut-off frequency for radiofrequency-field effects

Night-migratory songbirds use the Earth’s magnetic field to guide their migratory journeys. Most evidence suggests that the magnetic compass sensor is a flavin-tryptophan radical pair whose operation is disrupted by broadband radiofrequency (RF) fields at frequencies up to ∼116 MHz. Here, we test whether broadband 110–120 MHz RF fields affect the magnetic orientation behaviour of night-migratory Eurasian blackcaps ( Sylvia atricapilla ). We found that the birds oriented in their expected migratory direction in the natural geomagnetic field (NMF) control condition and that they turned their orientation ∼120° when the field was rotated 120° horizontally (changed magnetic field, CMF). When they were exposed to broadband 110–120 MHz RF fields, the birds continued to orient in the appropriate direction in the NMF. In the CMF, we found orientation behaviour that was not consistent with the expected direction. We conclude that the birds could still orient when exposed to 110–120 MHz fields, but speculate that their magnetic orientation capabilities might have been somewhat reduced compared to the control condition. We suggest that 110–120 MHz could represent a grey zone of reduced magnetic orientation capability centred at the cut-off frequency predicted for a flavin-based radical pair (probably in the flavoprotein cryptochrome).