Why do birds sing? In many species, because the song attracts or retains a mate. Why do females pay attention? This paper reviews evidence that females may do so because male song can be an honest indicator of attributes of a male's brain that could contribute to his fitness or that of his young. Male songbirds learn and produce their songs using a set of brain regions collectively known as the song system. The learning has distinct auditory and motor components, and current data suggest that the neural changes that encode these forms of learning primarily occur in different subdivisions of the song system. There are positive correlations between song complexity and the volume of motor song system nucleus HVC, both between and within species. The correlations appear to arise because individual differences in volume lead to differences in capacity for learning. The differences in HVC volume are correlated with differences in the volumes of other song system components and with the volume of the forebrain. They are heritable. Thus, a complex song can be a signal to a female of immediate fitness (the male has a larger brain) and ultimate fitness (he has attractive characteristics that will be passed on to progeny).
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