The Bengalese finch Lonchura striata var. domestica is a domesticated strain of a wild species, the white-rumped munia Lonchura striata of Southeast Asia. Bengalese finches have been domesticated in Japan for 240 years. Comparing their song syntax with that of their wild ancestors, we found that the domesticated strain has highly complex, conspicuous songs with finite-state syntax, while the wild ancestor sang very stereotyped linear songs. To examine the functional utility of the song complexity, we compared serum levels of estradiol and measured the amount of nesting materials carried into the nest by female birds that were stimulated with either the complex "domesticated" song or the simple wild-type song. In the females stimulated with complex songs the estradiol levels were significantly higher and the amount of nesting material carried was significantly greater. We then performed brain lesions in the song system to identify the neural substrates that are responsible for these differences in song behavior. In Bengalese finches lesions of NIf, a higher order song control nucleus, resulted in simplification of the complex song syntax. That is, the complex "domesticated" syntax changed into the simple wild-type syntax. Based on these data, we hypothesize that mutations in the song control nuclei have occurred that enabled complex song syntax and became fixed into the population of domesticated Bengalese finches through a process of indirect sexual selection.