Adult brains differ among species in the proportional sizes of their major subdivisions. For example, the telencephalon occupies 71% of the entire brain in parakeets (Melopsittacus undulatus) but only 54% in quail (Colinus virginianus). In contrast, the tectum is smaller in parakeets than in quail. To determine whether these differences in brain region size arise because of species differences in cell cycle rates, parakeet and quail embryos were collected at various stages of development (HH24–HH37) and stained with antibodies against proliferating cell nuclear antigen (PCNA), which labels all dividing cells, and phosphorylated histone-3 (pH3), which labels M-phase cells. Analysis of pH3+ cell densities and pH3+/PCNA+ cell ratios were used to compare cell cycle rates across stages and species. Cumulative labeling with bromodeoxyuridine (BrdU) was also used to compare cell cycle rates at stages 24 and 28 in quail. We found that telencephalic cell cycle rates lengthen with age in both species, but that they lengthen significantly later in parakeets than in quail. This species difference in cell cycle rates explains, at least partly, why adult parakeets have a proportionately larger telencephalon. Tectal cell cycle rates also remain elevated for a prolonged period of time in parakeets compared to quail. This seems paradoxical at first, given that the parakeet’s adult tectum is relatively small. However, the tectum is initially much smaller but then grows more extensively in parakeets than in quail. Thus, species differences in adult brain proportions can be traced back to species differences in cell cycle kinetics.