SEASONAL-CHANGES IN TESTOSTERONE, NEURAL ATTRIBUTES OF SONG CONTROL NUCLEI, AND SONG STRUCTURE IN WILD SONGBIRDS

Seasonal changes in the neural attributes of brain nuclei that control song in songbirds are among the most pronounced examples of naturally occurring plasticity in the adult brain of any vertebrate. The behavi oral correlates of this seasonal neural plasticity have not been well characterized, particularly in songbird species that lack adult song l earning. To address this question, we investigated the relationship be tween seasonal changes in gonadal steroids, song nuclei, and song beha vior in adult male song sparrows (Melospiza melodia). At four times of the year, we measured plasma concentrations of testosterone, neural a ttributes of song nuclei, and several aspects of song structure in wil d song sparrows of a nonmigratory population. We found seasonal change s in the song nuclei that were temporally correlated with changes in t estosterone concentrations and with changes in song stereotypy. Male s ong sparrows sang songs that were more variable in structure in the fa ll, when testosterone concentrations were low and song nuclei were sma ll, than in the spring, when testosterone concentrations were higher a nd song nuclei were larger. Despite seasonal changes in the song nucle i, the song sparrows continued to sing the same number of different so ng types, indicating that changes in the song nuclei were not correlat ed with changes in song repertoire size. These results suggest that so ng stereotypy, but not repertoire size, is a potential behavioral corr elate of seasonal plasticity in the avian song control system.