In zebra finches, only males sing and brain regions controlling song exhibit sex differences in neuron number that stem from actions of estrogen during a critical developmental period. In certain song nuclei, these dimorphisms emerge long after neurogenesis and migration are complete, and estrogen promotes masculinization by preventing the death of well-differentiated neurons. But in another region, the higher vocal center (HVC), cellular mechanisms underlying sex differences in neuron number are not so well understood. In the HVC, neurogenesis continues throughout the post-hatch period of sexual differentiation, and sex differences arise during this time because neuron number increases in males but not females. We used [3H]thymidine autoradiography to establish when sex differences in neuron number first develop among a small group of HVC neuronal cohorts. We report that HVC neurons labeled by [3H]thymidine on days 15 and 16 after hatching are sexually dimorphic in number within 10 days of their birth, even before all cells in this cohort complete their migration and/or differentiation. This suggests that the cellular mechanisms contributing to sex differences in neuron number in the HVC may differ from those in other sexually dimorphic neural regions of the vertebrate nervous system. In addition, we found that although many thymidine-labeled HVC neurons ultimately project to the robust nucleus of the archistriatum (RA), a sexually dimorphic target, sex differences in their number develop before this efferent projection is established. These results have important implications regarding the site(s) of hormone action, since they suggest that sexual differentiation acts on certain HVC neurons before they establish their efferent projections, and perhaps even before they arrive within the HVC.