Testosterone stimulated the production of soft song in female European robins. (A) Daily singing activity was recorded from testosterone (●) and time-matched control (○) birds. Testosterone pellets were subcutaneously implanted at day zero. (B-D) Sound waves (amplitude (analog/digital arbitrary units) over time (ms)) are shown in the upper panels and sonograms are presented in the lower panels (frequency (Hz) over time (ms)). For female robin # 14 (B) details of soft song after seven days of testosterone treatment and (C) first high amplitude song phrases after eleven days are depicted. In (D), a small portion of full song obtained from an adult male robin is depicted.