In human vocal production, the lungs produce energy for the voice, the vocal folds have a function of a sound source, and the vocal tract functions as a resonator. Birds have the syrinx for the sound source, but they do not have the vocal folds. In birdsong, the syrinx had been thought to act alone in song production, and vocal tract played no role. However, when a bird sings in helium, harmonic overtones emerge [Nowichi (1989)]. For this reason, it is suggested that the vocal tract may play an important role as a vocal filter in birdsong production. In this study, beam movements were measured in the Bengalese finches while they were singing, and the correlation between fundamental frequencies in the song and the beak gape was examined. This correlation was significantly high (r=0.890), the wider beak gape corresponding to a higher fundamental frequency. The result suggests that in Bengalese finches beak movement formulas a dynamic filter which modifies the frequency composition of the syringeal output. Thus sound production in songbirds may be comparable to that in humans.