Fig. 2. (i) Four video frames from a song sequence in a medium ground finch (Geospiza fortis). (ii) Sound spectrogram of the song produced. (iii) Beak gape profile, calculated from video frames and aligned with the sound spectrogram. Video frames A–D are indicated by broken vertical lines. Each syllable consists of three notes, the second of which is produced at slightly lower frequencies. These lower frequencies are matched by reduced gapes (e.g. frame C). Two movie files of this song sequence, the first at normal speed (`fortis.mov') and the second at one-third speed (`fortis-slow.mov'), are included online as supplemental material.