The developmental processes through which songbirds acquire their spec
ies-typical songs have been well-studied from a proximate perspective,
but less attention has been given to the ultimate question of why bir
ds learn to sing. We present a new hypothesis for the adaptive signifi
cance of song learning in songbirds, suggesting that this specialized
form of vocal development provides an indicator mechanism by which fem
ales can accurately assess the quality of potential mates. This hypoth
esis expands on the established idea that song earn provide an indicat
or of male quality, but it explicitly links the variation in song expr
ession that females use to choose mates to the developmental processes
through which song is acquired. How well a male sings-reflected in re
pertoire size or in other learned features of a male's singing behavio
r-provides an honest indicator of quality because the timing of song l
earning and, more importantly, the timing of the development of brain
structures mediating learning corresponds to a period in development d
uring which young songbirds are most likely to undergo nutritional str
ess. This correspondence means that song learning can provide a sensit
ive indicator of early developmental history in general, which in turn
reflects various aspects of the phenotypic and genotypic quality of a
potential mate.