In many songbirds, males vary aspects of their singing behaviour when engaged in territorial interactions. Song rate, song type switching rate, song matching, song overlapping, and the use of specific song or call types have all been proposed to be aggressive signals. It is not clear why such variability in aggressive signaling mechanisms exists among different species. We used a comparative approach to study how two Phylloscopus warbler species respond to playback-simulated territorial intrusion. We examined whether the spontaneous songs of Large-billed Phylloscopus magnirostris and Sulphur-breasted P. ricketii leaf-warblers differed from songs produced in response to playback. Song bouts were analysed by measuring 10–12 parameters, but we found no differences between spontaneous songs and playback responses in either species. All males clearly responded to playback by approaching the loudspeaker and flying around it. Large-billed Leaf-warblers produced ‘crackling’ sounds in response to playback more frequently than during spontaneous singing, whereas the usually highly vocal Sulphur-breasted Leaf-warblers possibly (and surprisingly) did not use vocalizations (either songs or calls) to signal direct aggression. A comparison with other Phylloscopus species revealed that even closely related species (i.e. from a single genus) might use different strategies when responding to simulated territorial intrusion. The aggressive signalling strategy is therefore a labile trait that can potentially be exposed to fast evolutionary change.