No abstract

When mobbing a predator, birds often produce specific mobbing calls that are efficient in recruiting both conspecifics and heterospecifics. Recent studies on Parids have demonstrated that these mobbing calls are in fact a combination of two distinct calls—first, introductory notes eliciting vigilance in the receiver, then broadband frequency notes (D notes) triggering approach. Debates on a parallel between human syntax and this form of combination have emerged. The degree to which this combinatoriality is perceived in heterospecific communication may shed light onto the relative complexity of such combinatoriality. In this study, our aim was to determine whether European great tits (Parus major) appropriately responded to mobbing calls (and their isolated parts) of an allopatric species, the black‐capped chickadee (Poecile atricapillus), a North‐American species which produces similar combinatorial mobbing calls. In addition, we tested whether the behavioural response to complete mobbing sequences was different than the simple sum of its two constituents. As we hypothesized, great tits behaved differently when hearing the two isolated calls or the complete mobbing sequence: they produced calls and displayed excitement signs only towards the complete mobbing sequence. Moreover, great tits responded to the introductory and D notes by respectively scanning and approaching, and to the complete sequence by mobbing. Our results altogether support the emerging hypothesis of semantic compositionality in Parids, although the present study does not definitively demonstrate the existence of this cognitive process in the great tit.

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 enlightens 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 StatementInformation 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 2 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 as 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.