The elaborateness of many bird songs is commonly presumed to have evolved under the influence of sexual selection by female mate choice. Thus, aspects of acoustic diversity, such as song repertoire size, are seen as likely targets of female choice. In many songbird species with song repertoires, however, the repertoires are small. In such species, female choice might be based on song features other than, or in addition to, song diversity. To investigate this conjecture, I assessed singing and paternity in a population of chestnut-sided warblers (Dendroica pensylvanica), a species in which song repertoires are of modest size. Twenty-two song traits were evaluated to determine which ones best predicted male extrapair reproductive success. The candidate traits encompassed measures of song diversity (e.g., song repertoire size), gross-scale song performance (e.g., singing rate), and fine-scale song performance (e.g., variability among songs in a bout). Regression analysis revealed that the best predictor of extrapair success was singing with little variability. In particular, the most successful males sang with consistent pitch and timing, as well as high pitch. The greater extrapair success of males with more consistent vocal performance may be due to female preference for such performance, which could be an indicator of male quality.