Electrophysiological responses and bootstrap results. a, c, e, g, h) Colored lines indicate ERP waveforms for each category. Black stars indicate 4 ms time bins at which electrophysiological responses to voices was significantly stronger than both the two other stimulus categories (voice > non-voice). Green circles indicate 4 ms time-bins at which ERPs bird songs were significantly greater than to both voice and environmental sounds. b, d, f) Headplot showing the voice > non-voice significant difference at 200 ms and electrode positions for panels a, c, g and h. Colormap indicates significant amplitude difference (in μV) with greater ERP amplitude (positive or negative) for voices compared to both bird songs and environmental sounds. Gray areas were not significant.