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. Given the large differences in social organization and environment throughout the year in passerine communities, the response to acoustic cues could be impacted by the season. In the present study, we tested the potential interaction between the number of callers, the species identity of the caller (i.e., coal tit or crested tit), and the season on birds mobbing response using a factorial playback experiment. 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. However, this effect interacted with the season: in spring, only soundtracks with three coal tits triggered a mobbing response. We therefore confirm the hypothesis that birds consider both the species and the number of callers when joining a mobbing chorus. Moreover, our study demonstrates how seasonal context interacts with acoustic cues and illustrates the dynamic properties of heterospecific communication.