Neurochemical specializations associated with vocal learning and production in songbirds and budgerigars.

Specialized neural circuitry has evolved in groups of birds where vocal learning is known to occur, such as in the oscine suborder of the Passeriformes (songbirds) and in the order Psittaciformes (parrots). These specializations are most prominent in the telencephalon, while the midbrain and medullary portions of the vocal control circuit are generally similar in all orders of birds. Specializations in songbirds have at least four components: 1) a set of distinct and interconnected vocal control nuclei in the forebrain; 2) unique connections between the auditory system and these vocal control nuclei; 3) the occurrence of receptors for sex steroid hormones in a subset of the vocal control nuclei; and 4) unique patterns in the distribution of various markers of the major classes of neurotransmitters within the vocal control nuclei. In the order Psittaciformes, as exemplified by budgerigars (Melopsittacus undulatus), it appears that neural specializations generally similar to those described in songbirds have evolved independently. Although anatomical studies have found a system of interconnected forebrain regions in budgerigars that are roughly similar to those described in oscines, detailed connectivity studies of this forebrain system suggest that it is only superficially similar to that of songbirds, and budgerigars also have unique connections between the auditory and motor systems. Also, analyses of the distribution of markers of neurotransmitter function in the budgerigar brain reveal patterns different from those described in songbird vocal control systems. This work suggests that songbirds and parrots have evolved separate neural 'solutions' to solve the problem of vocal plasticity. There are differences between these 'solutions' but also similarities that may be the result of convergent evolution. Although vocal behavior is learned in both songbirds and parrots, it differs in many respects. By taking advantage of this 'natural' experiment one can gain insight into the hormonal and neural events that mediate these different forms of vocal plasticity.