SYNOPSIS. The developmental processes through which songbirds acquire their species—typical songs have been well—studied from a proximate perspective, but less attention has been given to the ultimate question of why birds learn to sing. We present a new hypothesis for the adaptive significance of song learning in songbirds, suggesting that this specialized form of vocal development provides an indicator mechanism by which females can accurately assess the quality of potential mates. This hypothesis expands on the established idea that song can provide an indicator of male quality, but it explicitly links the variation in song expression that females use to choose mates to the developmental processes through which song is acquired. How well a male sings—reflected in repertoire size or in other learned features of a male's singing behavior—provides an honest indicator of quality because the timing of song learning and, more importantly, the timing of the development of brain structures mediating learning corresponds to a period in development during which young songbirds are most likely to undergo nutritional stress. This correspondence means that song learning can provide a sensitive indicator of early developmental history in general, which in turn reflects various aspects of the phenotypic and genotypic quality of a potential mate.