Abstract
Birdsong often serves as a territorial signal, and vocal performance (the degree of vocalizing close to vocal production or respiratory limits) can play a role in expressing a sender's aggressiveness. However, this is rarely studied in females, in which song has historically been considered non-functional. Here, we investigate male and female European robins, Erithacus rubecula, in which both sexes sing to defend their individual exclusive winter territory. We compared spontaneous song to song experimentally provoked by playbacks simulating territorial intrusions. Our study is the first to show that both sexes similarly increased vocal performance when territorially challenged, regardless of the simulated intruder's sex. This provides evidence for female song to be perceived as equally territorially challenging as male song and to similarly express aggressiveness. Additionally, females sang shorter songs than males, and in both sexes vocal performance was lower when singing longer songs, an effect that was stronger in females in one of our metrics. We propose that females might sing shorter songs to maintain a high level of vocal performance. This might be a strategy to cope with sex-specific effects of sustained singing on vocal performance.