Data from: Vocal performance in birdsong is an aggressive signal in both females and males: Experimental evidence from a field study in European robins

Birdsong often serves as a territorial signal, and vocal performance (the
degree of vocalising 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 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 that female song is perceived as equally territorially
challenging as male song and expresses aggressiveness similarly.
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.