An Hypothesis concerning the Relationship of Syringeal Structure to Vocal Abilities

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Abstract
Neither the possession of large vocabularies or repertoires nor the ability to learn phonations can be precisely correlated with the structural complexity of a syrinx. Hence, some recent investigators have suggested that avian vocal plasticity arises solely from a neurological shift. A simple syrinx, i.e. one with only extrinsic musculature, is subject to certain constraints, however. Its configuration changes as a unit, and the factors responsible for modulating sounds cannot be independently varied. Thus, the temporal characteristics of sound patterns can be varied easily, but rapid juxtaposition of different modulatory patterns is difficult. Intrinsic musculature permits isolation and independent control of syringeal components and thereby simplifies control of modulations. Syringeal complexity may not be an adaptation (i.e. did not evolve under selection) for plastic vocal behavior, but it is permissive of and probably prerequisite for such behavior.