Acoustic cues and season affect mobbing responses in a bird community

Heterospecific communication is common for birds when mobbing a predator. However, joining the mob should depend on the number of callers already enrolled, as larger mobs imply lower individual risks for the newcomer. In addition, some ‘community informant’ species seem more reliable regarding the information transferred in mobbing calls. Birds should therefore rely on both the number of callers and the species identity of the caller(s) when mobbing. In the present study, we tested the potential interaction between two acoustic cues (number of callers and the species identity of the caller). Overall, we found that soundtracks with three callers triggered more mobbing than soundtracks with one caller and that soundtracks with coal tits’ calls triggered more mobbing than soundtracks with crested tits’ calls. Our results therefore support the hypothesis that birds consider both the species and the number of callers when joining a mobbing chorus in winter. However, when the same experiments were replicated during spring, only soundtracks with three coal tits triggered a mobbing response, illustrating therefore how the seasonal context interacts can strongly affect the results of studies on heterospecific communication.