A two-signal, two-receiver signalling system: Snake mobbing in the Arabian babbler

Snake mobbing is a collective antipredator behavior observed in group-living Arabian babblers, involving two distinct signals: acoustic zwick calls and visual wing-lifting displays. Research on this behavior involved eliciting snake-mobbing events with dummy vipers and recording group interactions using a synchronized acoustic camera. This study reveals the unique and overlapping roles of these two signals in the Arabian babbler's multimodal mobbing behavior.

The recruitment order for the mobbing response was found to be independent of sex, age, and rank within the group. As group size increased, individual investment in signaling behavior decreased, supporting the hypotheses of social buffering and predation risk dilution. Singular individuals always called during wing-lifts, demonstrating the calls' primary recruitment function.

Individuals displaying both visual and acoustic signals responded faster to the joint visual-acoustic conspecific display than to an acoustic signal alone. When the two signals were combined, wing-lifts significantly altered vocal characteristics, including a decrease in call frequency and an increase in call rate, which strengthened the acoustic signal.

Furthermore, calls emitted at the end of the wing-lift display exhibited stronger vocal modulation. Overall, these findings highlight the multifunctional role of multimodal mobbing signals during a risky cooperative task, balancing social communication and predator deterrence.