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

Overview & Synopsis

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 com- binatorial 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.