Number of callers may affect the response to conspecific mobbing calls in great tits (Parus major)

Numerical competence—the ability to represent, discriminate, and process numerical quantity information—is a widespread cognitive ability in animals that influences survival and reproductive success. Little is known about the role of numerical competence during predator mobbing—when a prey moves toward and harasses a predator. Since being in a larger group dilutes the risk of injury or death during a mobbing event and large groups are more efficient than small groups at repelling predators, the capacity to evaluate the number of mobbers before joining the mobbing flock may be highly beneficial for individuals. We tested whether the strength of the mobbing response of great tits (Parus major), a songbird that frequently mobs predators, is related to the number of callers. The minimum distance to the loudspeaker tended to be lower, and the number of calls produced by great tits was higher during playbacks simulating several callers than during the playbacks of one caller. These results suggest that numerical competence plays a central role during mobbing and that great tits reduce uncertainty of information by collating information from several individuals. We suggest further studies testing whether birds use individual vocal discrimination to assess the number of heterospecifics during mobbing. SIGNIFICANCE STATEMENT: Can animals count? Although historically the ability to count has distinguished humans apart from the rest of the animal kingdom, studies in the last decades have shown that numerical competence, the ability to represent, discriminate, and process numerical quantity information, is a widespread cognitive ability in animals. While this competence influences an individual’s survival success, little is known about the role of numerical competence during predator mobbing. Using a field-based playback experiment on a population of wild great tits (Parus major), we demonstrate that great tit responses to mobbing calls were affected by the number of individuals calling. The minimum distance to the loudspeaker tended to be lower and the number of calls produced by great tits tended to be higher during playbacks simulating multiple callers than during the playbacks of one caller. Thus, numerical assessments are used to decide whether or not to participate in mobbing responses.