The ability to communicate predator category and the risk posed by the predator to conspecifics would be adaptive since it would allow prey to adjust their mobbing and escape behaviour to the specific risk posed by different predators. However, it remains unclear so far if mobbing calls, not only in meerkats but also in other species, denote the risk posed by the predator and predator category. In meerkats Suricata suricatta, mobbing calls vary depending on predator category as well as risk and conspecifics adjust their predator inspection behaviour accordingly ( Manser et al.

Field observations and experiments have shown that prey utter different calls, or alter the number of times calls are given, when approaching different predator categories ( Naguib et al. Second, mobbing calls can inform other group members and convey information about the risk posed by the predator or level of urgency to conspecifics ( Naguib et al. First, predators are sensitive to disturbance and move on as a reaction to mobbing prey ( Kruuk 1964 Pavey & Smyth 1998), and laboratory experiments demonstrated the distressing effect of mobbing calls on predators ( Shalter 1978). While mobbing can be costly, approaching predators has been suggested to be beneficial for at least two reasons. Many species of prey gather to approach and inspect or even harass resting predators ( Owings & Coss 1977 Pavey & Smyth 1998 Caro 2005), although these behaviours are not risk-free because prey can be taken by the predator ( Dugatkin & Godin 1992). However, predators are not hunting all the time and spend a substantial amount of their time resting. Alternatively, many prey have evolved behavioural strategies, such as grouping together or warning call systems, to reduce predation risk ( Caro 2005). Since encounters with a predator put the life of prey at risk, prey often rely on morphological adaptations, such as cryptic coloration, to reduce the likelihood of being detected ( Caro 2005). Predation is an important evolutionary force shaping life histories and behavioural decisions of prey. Since antipredator calls of Siberian jays are known to specifically aim at reducing the risk to relatives, kin-based sociality could be an important factor in facilitating the evolution of predator-category-specific mobbing calls. Thus, this is one of the first studies to demonstrate that mobbing calls can simultaneously encode information about both predator category and the risk posed by a predator. Five call types were predator-category-specific and jays uttered two hawk-specific and three owl-specific call types. Jays gave more calls when mobbing a more dangerous predator and when in the presence of kin. Groups immediately approached to mob the mount and uttered up to 14 different call types. I tested this idea in Siberian jays Perisoreus infaustus, a group-living bird species, by exposing jay groups to mounts of three hawk and three owl species of varying risks.

The ability to communicate predator category would be adaptive because it would allow other mobbers to adjust their risk taking. However, the risk posed by predators also differs between predator categories. While mobbing, prey usually utter calls and previous experiments have demonstrated that mobbing calls can convey information about risk to conspecifics. Many prey species gather together to approach and harass their predators despite the associated risks.

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