Conflicts about resources can be solved with physical fights, but animals will in general try to avoid them due to the costs and risks associated with fighting. Fight outcome depends on the fighting ability as well as the motivation to fight, which means that individuals should try to assess both during a pre-escalation phase. In the present study, we investigated whether blackbirds (Turdus merula) acoustically signal their motivation to fight with their advertisement song, which consists of a motif part with pure-toned low frequency elements followed by a twitter part with more complex and higher-pitched elements. We tested (1) whether the temporal and spectral characteristics of spontaneous songs broadcasted by a territorial male could predict its subsequent aggressive behaviour to a playback and (2) if the song of territorial males changed in case they responded aggressively to the playback. We found no effects in the temporal song structure, which contradicts results from a previous study involving playback experiments with blackbirds. We did however find an interesting pattern in the spectral characteristics of the twitter, since aggressively responding males significantly increased the twitter frequency after the playback. The post-playback songs of aggressive responders were nevertheless indistinguishable from spontaneous pre-playback songs of non-aggressive responders. Therefore, we conclude that changes in twitter frequency, rather than absolute twitter frequencies, are advertising information about motivation to fight.