Fig. 4. Acoustic correlates of opening and shutting the beak. (A) Differences in fundamental frequency for 14 syllable pairs (two syllables or portions of syllables from the same song with similar acoustic structure but differing in beak aperture, see Fig. 2 for examples). Error bars (± S.E.M.) for the fundamental frequency were generally so small as to be contained within the symbol, and fundamental frequency did not differ for the two beak positions. (B) Peak frequency (the frequency with the highest energy) was more variable, and showed a significant overall reduction when the beak was closed, although two syllables did increase in peak frequency. (C) The average relative amplitude also decreased significantly overall when the beak was closed (although three syllables increased in amplitude in this condition). (D) These trends are illustrated in two neighboring syllables from DP46’s song (also pictured as syllables Y and Z in Fig. 3B, but note that the beak trajectory for these syllables does not appear in that figure because the syllables were absent from the father’s song). Beak aperture was reduced during the first syllable and increased during the second syllable. The two syllables have nearly identical fundamental frequencies, but the second syllable, when the beak was relatively open, had a higher amplitude (as is apparent in the oscillogram and in the overall ‘darkness’ of the sonogram). The energy in the second syllable is concentrated in higher harmonics than for the first syllable; compare the fifth and sixth harmonics for the two syllables. Note that, because change in beak aperture and not the beak aperture itself was measured, the two beak positions (open and closed) are relative.