Radio frequency noise measurements from Migratory bats are sensitive to magnetic inclination changes during compass calibration period

To test whether the coil, or specifically the vertical axis of the coil, was producing RF noise that may differ between the treatments in our experiment we used a single wrapped Helmholtz coil (the same model used in the experiments in Latvia) and a passive antenna placed either vertically (Figure S4) or horizontally (Figure S5) connected to a spectrum analyser (MP700022 EU-UK). The coil was either unpowered (black trace), turned on with the power settings matched to those used in the 120 degree shift positive inclination treatment used in our experiment (red trace), or turned on with the power matched to the negative inclination treatment. The spectrum analyser was set to record ‘Max Hold’ – collected the maximum power readings at all frequency bands during a 30 second window. Whist there was clearly the presence of background RF signals (e.g. the 100 MHz spike can be attributed to FM radio signals), these were present at the same level in each of the recordings we took, with no differences that may suggest RF noise produced by the coil may have caused the differences that we observed in our treatments.