The caudal nucleus of the ventral hyperstriatum (HVc) and the robust nucleus of the archistriatum (RA) are two anatomically discrete brain regions that are known to be involved with song production in adult passerine birds. Both the HVc and RA increase greatly in volume during a restricted period of song development in male zebra finches, while brain regions not involved with song control show little or no increase in size. We report here that the increased volume of the HVc is attributable to an increase in the number of neurons during this period of song learning, whereas the growth of the RA is due to an increase in the somal size of neurons and a decrease in neuronal density. The pattern of results described is consistent with the idea that the HVc matures prior to the RA, and that the development of the RA may depend on the ingrowth of axons from the HVc and other song-control regions.