Sexual dimorphism and lack of seasonal changes in vocal control regions of the white-crowned sparrow brain.

The volumes of brain regions involved in vocal control were measured in adult male and female white-crowned sparrows (Zonotrichia leucophrys nuttali) captured in the summer, and in captive males held on long-day or short-day photoperiods. There is a large sex difference in the volume of two nuclei, the caudal nucleus of the hyperstriatum ventrale (HVc) and the robust nucleus of the archistriatum (RA), which correlates with a large sex difference in singing behavior. There were no differences in the size of HVc and RA in adult males held on summer or winter photoperiods, even though the 'summer' males had high androgen levels and were singing, whereas the 'winter' males had regressed testes and were not singing. The data bear on hypotheses concerning the relationship between size of brain nuclei and song learning.