The warble song of budgerigar (Melopsittacus undulatus) contains various classes of vocal elements, including contact calls, alarm calls, harmonic calls, etc. Previous work has shown that budgerigars can discriminate different call types according to both acoustic and functional features; however, there is no evidence on how well birds can distinguish subtle differences within and between each category. Using operant conditioning and psychophysical methods, budgerigars were trained to discriminate among different warble elements. In general, birds were able to discriminate quite easily between all warble elements, even those with high spectral cross correlations. Humans, on the other hand, had difficulty distinguishing among warble elements that were similar to one another. The relative discriminability of warble elements within and across categories is used to more precisely define the functional categories of warble elements in understanding vocal communication in this species. [Work supported by NIDCD DC000198 to R.J.D. and P30 DC004664.]