Population differences in complexity of a learned skill are correlated with the brain space involved.

The song of marsh wrens (Cistothorus palustris) is a learned trait passed on from generation to generation. Male marsh wrens from California and New York learn about 150 and 50 different songs apiece, respectively. The volumes of the hyperstriatum ventralis, pars caudalis and the robust nucleus of the archistriatum, two telencephalic nuclei involved in song control, are larger by an average of 40% and 30%, respectively, in the population with a larger song repertoire.