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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 AD 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.