Which acoustic parameters modify the great tit's response to conspecific combinatorial mobbing calls?

When responding to a conspecific call, especially when mobbing a predator, receivers rely on acoustic cues produced by callers. Variation in calls, either gradual or discrete, can inform potential mobbers about the situation. Great tits use a combinatorial call made of frequency-modulated elements (FMEs) and D notes. The gradual properties of the D notes change when facing different situations (e.g., different predators). We tested the relative effect of the number of D notes per call or the number of calls per minute on the behaviour of great tits with a playback experiment on free-ranging tits. Great tits did not change their approach behaviour depending on the treatments broadcast. However, they increased their vigilance level with an increased number of D notes in the total sequence. We propose that great tits can consider the overall amount of D notes as a form of information about the predator being mobbed. In addition, an increased number of D notes did not modify the approach behaviour but modified the vigilance behaviour, possibly indicating that great tits process the complete FME-D call as a whole mobbing unit rather than the simple sum of a vigilance call (FME) and a recruitment call (D notes). Our study sheds new light on the coding system of great tits when mobbing a predator and, more generally, how animals can combine simple syntactic rules and gradual variations when communicating. Significance statement Information transfer when communicating about a predator is well described, particularly when mobbing a predator. However, it is unclear which specific variable(s) receivers rely on when adapting their response. Here, we test whether great tits adapt their response when the number of calls per minute and/or the number of notes per call is increased. Our results indicate that great tits rely on the overall number of notes in a calling sequence as a proxy for urgency. We also propose that the combinatorial call they use when mobbing is considered a whole rather than the simple sum of its constituents. Understanding how information transfer occurs in birds using combinatorial calls instead of simple note repetition adds to the current insights about compositional syntax in animals.