The vocal organ (syrinx) of a bird may contain either extrinsic muscles alone or both extrinsic and intrinsic muscles. The former arise and insert on the trachea and affect the syrinx only indirectly; the latter also arise on the trachea but insert directly on syringeal elements. It is widely supposed that syringeal muscles can affect modulations of the sounds the birds make, and further, that the intrinsic muscles are closely associated with such a function. However, the exact roles of the two groups of muscles have not been directly observed. The psittacid syrinx, which has one (for practical purposes) pair of extrinsic and two pairs of intrinsic muscles, is about as simple as one can find in birds capable of uttering a wide variety of sounds. We have taken electromyograms from the syringeal muscles of five species of parrots. In all of these, the extrinsic sternotrachealis showed the simple “activation” pattern activity previously described from several non-passerine species that possess only extrinsic muscles. The intrinsic muscles, however, showed a variety of activity patterns. The relatively simple call of Cyanoliseus patagonus again showed the simple activation pattern. In Myiopsitta monachus, the muscles showed a string of pulses that matched to pulses of sound in a strongly amplitude modulated call. Agapornis roseicollis used at least two distinct patterns, each associated with a different call. The results are consistent with an hypothesis that, because of their indirect attachment of the syrinx, extrinsic muscles are poorly suited to the production of precise, rapid changes in syringeal action, but rather will function in an “on-off switch” capacity. Intrinsic muscles are so situated that, given proper neurological stimulus, they can effect a variety of alterations in the sound pattern. Hence, intrinsic muscles are necessary for the evolution of large vocabularies and variable vocal behavior.