Fig. 1. (A) Illustration of how a variable frequency resonance filter might `track' fundamental frequency (solid vertical lines) as beak gape increases (as conceived by Hoese et al., 2000). (B) Illustration of how a filter with a single cut-off frequency near 4 kHz that is controlled by beak gape (solid lines) might function to attenuate tracheal resonances (broken lines) that occur above 4 kHz. Beak gape increases from the left graph to the right graph.