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Parrots, including parakeets, evolved significantly larger brains than other birds, relative to their body size, and they possess a proportionately larger telencephalon. For example, the telencephalon occupies approximately 68% of the brain in parakeets but only 52% in bobwhite quail. The present study was designed to determine when and how this difference in brain region proportions arises during development. To that end, we present volumetric data on the major brain regions in parakeets and bobwhite quail at several stages of embryogenesis, at hatching and, for the parakeets, 1 week after hatching. We also report on the proportional sizes of each region's proliferative and postproliferative zones. One major finding is that parakeets develop a proportionately larger telencephalon relatively late in development and that this late increase correlates with a delay in telencephalic neurogenesis. The most prominent …