A new study says crows have evidenced brain function for vocalization.

…volitional vocalizations carry specific cognitive control demands, previously undescribed in birds…

The first call after a run of pecks came late; by the second call the delay was gone. Pecks after calls showed no delay at all.

In other words, a vocal channel has a control layer that the beak-peck channel does not. Calling is cue-to-decision-to-action. Pecking is cue-to-action.