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First published online January 12, 2004
Journal of Experimental Biology 207, 607-619 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00770

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Vocal mechanics in Darwin's finches: correlation of beak gape and song frequency

Jeffrey Podos^1,*, Joel A. Southall¹ and Marcos R. Rossi-Santos²

¹ Department of Biology, University of Massachusetts, Amherst, MA 01003, USA
² Projeto Baleia Jubarte, Rua 7 de Setembro 178, Caravelas, Bahia 45900-000, Brazil

View larger version (52K): [in a new window]   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.

View larger version (54K): [in a new window]   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.

View larger version (32K): [in a new window]   Fig. 3. Representative sound spectrograms and gape profiles for six species in this study. Changes in gape generally correspond to shifts in song frequency.

View larger version (25K): [in a new window]   Fig. 4. Sound spectrograms, gape profiles and gape x frequency plots for three Geospiza fortis individuals. These birds represent the range of within-species variation in regression slopes and y-intercepts; our sample varied significantly in y-intercepts and especially slopes. We failed to detect any significant correlations, however, between regression parameters, song parameters and age (as indicated by plumage – see text).

View larger version (19K): [in a new window]   Fig. 5. Song frequency regressed onto beak gape for seven Darwin's finch species.