The number of Great Tit mobbers influences the mobbing response of heterospecific birds

Many species mob predators to drive them away. Mobbing carries personal risk, but the risk of injury or death declines and the likelihood of repelling the predator increases in larger groups. The capacity to evaluate the number of mobbers before joining a mobbing group may be highly beneficial for individuals when deciding to join. Although recent studies have found that birds can use individual vocal discrimination to assess the number of conspecifics involved in initiating mobbing events, it is little known whether birds are able to evaluate the number of heterospecific mobbers. In this study, we investigated whether the number of heterospecifics responding to mobbing calls of Great Tits Parus major was influenced by the number of callers (a mix of Great Tit callers simulated by playbacks and live Great Tits attracted to playbacks, range one to seven callers). The total number of responding heterospecific individuals and species was positively influenced by the total number of callers, with heterospecifics responding more to larger than smaller Great Tit groups. However, these results may be driven by the total number of calls, call overlap and increased noise generated by the additional mobbers, and not the number per se. These findings therefore allow us to assess whether birds are more inclined to join a mobbing group when it consists of more heterospecific callers, but we cannot tell whether birds use individual vocal discrimination to assess the number of heterospecific callers.