Acoustic signalling mediates key animal behaviours, but the factors driving inter- and intra-specific variation remain elusive, especially in non-learning suboscine birds. Acoustic traits may evolve to signal an individual’s quality through a process termed honest signalling. Most studies on bird song evolution via honest signalling focus on oscine bird species (song learners) while fewer evaluate honest signalling in suboscines. We tested whether song was an honest signal of male quality in southwestern willow flycatchers (Empidonax traillii extimus), a suboscine, by monitoring nest success and recording male advertisement songs. Based on song characteristics known to vary among individuals, we found higher minimum song frequency, but not temporal components, was positively correlated with the number of fledglings and nest success, after accounting for first egg date, the number of eggs, and clutch number. Our study therefore offers evidence that suboscine song may have evolved as an honest signal of reproductive success.
