Fig. 4.

Heparitinase treatment leads to the loss of high-frequency syllables. Examples from 2 different heparitinase-treated birds (A and B) show the complete or partial loss of high-frequency syllables at 24 h after injection (the highlighted areas indicate the segment during which the high-frequency sounds are generated; P, air-sac pressure; S, oscillogram and spectrogram, bottom panels; F, frequency). To allow direct comparison, the spectrograms were calculated with the same dynamic range. The components of high-frequency syllables lost 24 h after the enzyme treatment have reappeared by 7 days (d) after injection, indicating this loss is temporary. This effect occurs in high-frequency syllables that are generated during inspiration (A) and expiration (B).