Dooling, R.J., Soli, S.D., Kline, R.M., Park, T.J., Hue, C., and Bunnell, T. (1987). Perception of synthetic speech sounds by the budgerigar (Melopsittacus undulatus). Bulletin of the Psychonomic Society, 25, 139-142.

Two budgerigars were trained to respond differently to two synthetic speech tokens, /da/ and /ta/. Voice onset times (VOTs) of these synthetic stimuli were 0 and +70 msec, respectively. After reaching criterion level performance on these stimuli, the budgerigars were tested with synthetic stimuli having VOTs between 0 and +70 msec. The categorization functions derived from each bird's responses to these intermediate stimuli were similar in shape but differed in their boundary locations from the functions typically obtained from humans and other mammals. In addition, the birds found this speech discrimination task extraordinarily difficult. These results define some of the limitations of the budgerigar's auditory system for the processing of complex acoustic stimuli. Since the budgerigar has a distinctly nonmammalian auditory system, these results also have relevance for current theories of speech perception.