Caller characteristics influence recruitment to collective anti-predator events in jackdaws (original) (raw)

Across the animal kingdom, examples abound of individuals coming together to repel external threats. When such collective actions are initiated by recruitment signals, individuals may benefit from being selective in whom they join, so the identity of the initiator may determine the magnitude of the group response. However, the role of signaller discrimination in coordinating group-level responses has yet to be tested. Here we show that in wild jackdaws, a colonial corvid species, collective responses to anti-predator recruitment calls are mediated by caller characteristics. In playbacks next to nestboxes, the calls of nestbox residents attracted most recruits, followed in turn by other colony members, noncolony members and rooks (a sympatric corvid). Playbacks in fields outside nestbox colonies, where the immediate threat to broods was lower, showed similar results, with highest recruitment to nearby colony members' calls. Responses were further influenced by caller sex: calls from non-colony member females were less likely to elicit responsive scolding by recruits than other calls, potentially reflecting social rank associated with sex and colony membership. These results show that vocal discrimination mediates jackdaws' collective responses and highlight the need for further research into the cognitive basis of collective actions in animal groups. From army ants to human armies, groups of conspecifics across many taxa exhibit collective responses towards external threats, often in response to specific recruitment signals 1-4. The success of defensive groups is likely to be closely linked to their size 5 , so it is important to understand the processes that determine the magnitude of collective responses. Group defences often present collective action problems, in which individuals face conflicting incentives when deciding whether to join 6,7. In anti-predator mobbing, for example, joining a mob can provide a collective benefit by helping to drive away a potentially lethal threat 8,9 , and individuals may also benefit from gathering information about the predator 2,3. On the other hand, approaching a predator may be highly risky, particularly for individuals in small groups, creating an incentive to defect from joining the mob and free-ride on others' efforts 5,7,10. Under these circumstances, the identity of the initiator may provide crucial information to individuals deciding whether to join the mobbing group, thus influencing the magnitude of the group response. In many species, alarm vocalisations could provide an important cue to initiator identity. However, most research to date research has focused on alarm calls that elicit individual evasive behaviour rather than collective mobbing. If alarm calls signal an imminent and severe threat, failure to respond could be fatal, so individuals may benefit from responding with evasive action regardless of who produced the call. Meerkats (Suricata suricatta), for example, live under high predation pressure and, although their alarm calls are individually distinctive, their responses are unaffected by the identity of the caller 11. Caller characteristics may be more likely to influence alarm responses if the level of risk is relatively low, or particular categories of individuals are especially vulnerable. Yellow-bellied marmots, Marmota flaviventris, for example, respond more strongly to the alarm calls of vulnerable juveniles than those of adult females, but do not appear to discriminate between the calls of different individuals within the same age/sex category 12. Individual vocal discrimination could be advantageous if some callers are unreliable. Vervet monkeys (Chlorocebus pygerythrus), for instance, cease to respond the inter-group alarm calls of individuals that appear (through repeated playbacks of their calls) to "cry wolf ", calling there is no other group is present 13. Such selective responses are not apparent, however, in the higher-risk context of anti-predator alarm calls. Here, group members habituated to a particular individual's leopard-specific alarm calls nevertheless showed strong responses to that same individual's eagle-specific call 13. These results suggest that threat level plays an important role in determining the impact of individual caller identity on receivers' responses to alarms.