Birdsong is an acoustic ornament. According to indicator models, a trait must be costly to act as an honest signal, but the potential costs of elaborate songs are still poorly understood. The developmental stress hypothesis suggests that learned song characteristics could be an honest indicator of early developmental conditions because the brain structures associated with learning songs are susceptible to early developmental stress, which could thus affect song development. Unlike previous studies of developmental stress that examined the effect of a stress hormone or restricted nutrition, we observed Bengalese finches under semi-natural breeding conditions in captivity to investigate the relationship between early rearing conditions (e.g., brood size and sex ratio) and the subsequent variation in body size and song among individuals. Our results suggest that the early rearing environment directly affects body size and song complexity, whereas song output is determined mainly by body size. These results support the developmental stress hypothesis. Moreover, our findings are the first to show that developmental condition affects not only the number of note types but also the syntactical complexity of the song.