The bird's vocal tract, like the human vocal tract in speech, acts as a resonance filter that can control the sound coming from the mouth. Beak movements during song also contribute to this filter, but are not as important as changes in the size of the internal vocal tract. Human sopranos use the same technique as the cardinal to increase the loudness of very high notes so they can be heard above the orchestra. |
We show that songbirds adjust the size and shape of their vocal tract to 'fit' the changing frequency of their song. This enables the bird to produce a more whistle-like, pure-tone song. |