<p>Acoustic signals are central to bird territorial interactions, yet the role of song frequency in signalling competitive ability remains debated. Frequency has often been proposed to reflect body size through morphological constraints, but evidence within species is inconsistent, raising questions about whether it reliably informs rival assessment. Here, we tested the role of song frequency in territorial interactions in a non-learning bird species, the Emerald-spotted Wood-Dove <i>Turtur chalcospilos</i>, in which song frequency is highly consistent within individuals but only weakly correlated with structural body size. Using playback experiments, we presented territorial males with either higher- and lower-frequency versions of conspecific songs and quantified their behavioural responses. Males were less likely to approach lower-frequency playbacks and showed reduced movement-based responses as the simulated intruder’s song became progressively lower in frequency relative to that of the focal male. These results show that song frequency influences territorial behaviour and is used during rival assessment. Although song frequency is only weakly associated with structural body size in this species, our experiment does not identify what information this signal encodes. Instead, it suggests that the role of frequency in territorial assessment cannot be explained by measured body size alone and may involve additional morphological, biomechanical, or perceptual mechanisms. Overall, our study provides experimental evidence that song frequency contributes to territorial decision-making and adds to the ongoing debate over the role of frequency in competitive interactions.</p>