In seasonally breeding adult male songbirds, the volumes of several song control regions (SCRs) change seasonally in parallel with plasma testosterone (T) levels and decrease following gonadectomy. Testosterone treatment to castrates prevents this decrease, indicating T dependency. During the breeding season, second-year (SY: birds entering their first breeding season) free-ranging male Dark-eyed Juncos (Junco hyemalis) have smaller testes than older (after second-year, ASY: birds entering at least their second breeding season) birds. SY males also have lower plasma T concentrations than ASY males at the beginning of the breeding season. We investigated differences in song structure of the two age groups and the relationship between age differences in gonadal function and SCR sizes. The average number of syllables per song, syllable duration, trill rate, song duration, and variability in song duration were age-independent. Two brain regions that are thought to be involved primarily in song learning and perception were 13 and 18% larger, respectively, in SY than in ASY males, the opposite of what would be expected based solely on reproductive measures (testis mass and cloacal protuberance width). In contrast, the volumes of two regions that directly control song expression did not differ with age. The lack of age-related size differences in regions that are required for song production may indicate that male juncos of all ages have similar brain space requirements for motor production. Where there were size differences, they were restricted to regions primarily controlling vocal behavior acquisition/perception, suggesting that first time breeders need more brain space than experienced breeders to acquire crystallized song and/or acoustically perceive aspects of their environment.

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