The ability to perform physically challenging songs predicts age and size in male swamp sparrows, Melospiza georgiana

Selection should favour female preferences for reliable signals of male quality when such preferences result in benefits to females. Research on bird song suggests that, because song is costly to produce or sustain, females are obtaining accurate information about male quality through song preferences. Females have been shown to express mating preferences for three general categories of costly song features: song output, song complexity and geographical variation. A novel mechanism for the reliability of song is suggested by constraints on the ability to produce rapid, broadband trills (i.e. ‘vocal performance’). In several species of birds, females show a preference for superior vocal performance, supporting a key prediction of the hypothesis that vocal performance, like other features of song, may be a reliable indicator of male quality. In this study, I further test this hypothesis by investigating whether female swamp sparrows' preference for vocal performance is favoured by selection because it reliably reflects male quality. I found that vocal performance in male swamp sparrows was correlated with age and size, measured as mass. By preferring males with superior vocal performance, female swamp sparrows may obtain direct and indirect benefits by mating with older larger males. These results support the hypothesis that vocal performance in swamps sparrows is a reliable indicator of male quality.