From army ants to human armies, groups of conspecifics across a range of taxa exhibit collective responses towards external threats, often in response to specific recruitment signals. Such collective behaviours have become a major focus of research but the mechanisms underlying them remain poorly understood. Recruitment signals can convey important information about the nature of the threat, but also about the identity of the signaller. As joining in collective anti-predator responses is risky, individuals may benefit from being selective in whom they join. However, while a few studies have shown selectivity in individuals’ evasive or vigilant responses towards alarm calls, the role of vocal discrimination in coordinating group-level responses such as mobbing has yet to be tested. Here we show that in wild jackdaws (Corvus monedula), a colonial corvid species, collective anti-predator mobbing is mediated by initiator identity. Playbacks of anti-predator recruitment calls at nest-boxes generated positive feedback loops whereby recruitment increased when recruits made recruitment calls of their own. Playbacks of nest-box residents elicited most recruits, followed in turn by other colony members, non-colony members and rooks (a sympatric corvid species).  Similarly, playbacks conducted in nearby open fields, where the immediate threat to broods was lower, showed greatest recruitment responses to colony members’ calls, while the calls of non-colony females (which are subordinate to males) attracted the fewest recruits. These results show that vocal discrimination mediates jackdaws’ collective responses and highlight the need for further research into the cognitive basis of collective behaviour.