SignificanceBillions of small migratory songbirds fly thousands of kilometers every year between their breeding and wintering grounds, navigating with the help of an extraordinary ability to detect the direction of the Earth’s magnetic field. In behavioral tests, this ability can be disrupted by exposing birds to weak time-dependent electromagnetic fields. An important unknown quantity, which would provide vital insight into the poorly understood magnetic sensory mechanism, is the maximum frequency at which such disruption occurs. For Eurasian blackcaps, this frequency is found to lie between ~80 MHz and ~145 MHz in agreement with theoretical predictions, providing strong support for a radical-pair–based magnetic sensing mechanism.