Fig. 1. (i) Four video frames from a song sequence in a large tree finch (Camarhynchus psittacula). The bird adjusts beak gape during the production of this song. This bird also appears to adjust vocal tract volume using head extension (frame A) and retraction (frame B). (ii) Sound spectrogram of the song produced. Rapid and large-scale changes in fundamental frequency are evident. (iii) Beak gape profile, calculated from video frames and aligned with the sound spectrogram. Video frames A–D are indicated by broken vertical lines. The birds' gape is 0.4 cm at low frequencies (frames A and C), 0.8 cm at high frequencies (frame B) and nearly closed at song completion (frame D). Two movie files of this song sequence, the first at normal speed (`psittacula.mov') and the second at one-third speed (`psittacula-slow.mov'), are included online as supplementary material.