The total number and packing density of HVC-RA neurons increases between 90 d and 11 years of age in the adult male zebra finch whereas the number and density of HVC-X neurons do not change over the same age range. a, There is a significant increase in the total number of HVC-RA projection neurons between 90 d and 11 years of age (p < 0.0005), but there is no change in the number of HVC-X projection neurons over the same age range (p = 0.19). b, HVC volume does not change with age between 90 d and 11 years (p = 0.67). c, The packing density of HVC-RA neurons in HVC significantly increases between 90 d and 11 years of age in the adult male zebra finch and this relationship is best modeled using logarithmic regression (p = 1.8 × 10−10). The density of HVC-X neurons does not change with age over the same age range (p = 0.56). For all three graphs, each data point represents the average of the left and right HVC from one bird.