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Many animals produce vocal signals during agonistic interactions that convey their ability and motivation to escalate the conflict. Often, such disputes follow ritualized, sequential phases with increasing aggression, favouring the assessment of individuals' chances of winning. This allows for individuals to withdraw before getting involved in a serious fight. Birds are well known for the variety of information encoded in their vocalizations, sometimes including specific features that signal aggression. A particular case may be so-called 'tret calls' produced by chiffchaffs, Phylloscopus collybita , a song affix for which the communicative role is not well understood. Here, we describe the acoustic structure and use of tret calls during simulated conflicts using playback. Our results show that tret calls are a low-amplitude vocalization and that birds increased tret calling after playback stimulation, suggesting a role as an aggressive signal similar to soft song in other species. However, we did not find that the tret call rate predicted subsequent aggressive behaviour. Our analysis of song variation in response to playback suggested that birds respond to a simulated vocal competitor in two phases: a highly aggressive phase where syllable rate goes up, which is predictive of physical attack, while simultaneously tret call rate goes down, and a low-aggression phase, in which tret call rate goes up, while syllable rate returns to baseline levels. These stereotypic fluctuations in two acoustic features may be part of an advanced communication system to moderate aggression. Finally, our results show a seasonal increase in tret calling after birds arrive at their breeding grounds, which is in line with a role in territorial aggression. We also found a nonsignificant trend for more tret calls with increasing ambient noise levels, which may be due to increased stress and aggression under noisy conditions or reflect a masking avoidance strategy. • A call affix to the song of chiffchaffs, tret calls, may be a signal of aggression. • Tret calling increased during the breeding season and with ambient noise levels. • Tret calls in song did not predict approach but increased during simulated contest. • We propose that tret calls form part of an escalated system of aggression. • Low aggression phase during the contest may be signalled by increased tret calling. [ABSTRACT FROM AUTHOR]

All songbirds have their own species-specific song, and vocal variety among individuals of the same species is used for communication. Some aspects of vocal variety have been shown to relate to sender characteristics and thus to convey a potential message to receivers. During playback experiments, individuals show different response patterns, which provide evidence for perception, and thus meaning, of the vocal variety. Here, we tested the impact of two types of vocal variety: syllable diversity and syllable switching in the common chiffchaff, Phylloscopus collybita , in two separate playback experiments. We found that syllable diversity was a relatively fixed trait and that variation among individuals was likely to reflect some male quality triggering responses of different intensity. Higher rates of syllable switching did not elicit different responses but songs after playback showed that it is a dynamic trait, potentially contributing to motivational signalling together with other song parameters. The resolution of analyses in our experiments revealed subtle changes in vocal features over time in unprecedented detail. This approach unveiled an intricate combination of various vocal features during the response that may complement each other during vocal interactions. We believe future studies would benefit from the same resolution, through which one can explore advanced levels in animal communication. • Syllable diversity is likely to be a fixed trait signalling male quality. • Syllable switching may be a dynamic trait related to motivation. • Birdsong is highly dynamic during vocal interactions. • Some parameters follow opposite patterns during vocal interactions. • Studying behaviour at high temporal resolution offers advanced insights. [ABSTRACT FROM AUTHOR]