Caller characteristics influence recruitment to collective anti-predator events in jackdaws

Richard D Woods,Alex Thornton,Michael Kings,Guillam E Mcivor

doi:10.1038/s41598-018-25793-y

Abstract

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, non-colony 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.

Highlights

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[1,2,3,4]
Selective responses to different callers may be likely if responding to a call increases rather than reduces an individual’s exposure to threats. This is the case in collective anti-predator mobbing responses, where individuals that respond to recruitment calls move towards the threat, placing themselves in more danger than if they did not respond[5,7]
We used playback experiments on wild jackdaws (Corvus monedula) to investigate whether collective responses to anti-predator recruitment calls depend on caller identity

Summary

Introduction

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[1,2,3,4]. Selective responses to different callers may be likely if responding to a call increases rather than reduces an individual’s exposure to threats This is the case in collective anti-predator mobbing responses, where individuals that respond to recruitment calls move towards the threat, placing themselves in more danger than if they did not respond[5,7]. Given that initial recruits may themselves be followed by others, and that the costs of joining a mobbing event should decline as the number of recruits increases[5], selective responses to initiators could, in principle, have substantial effects on the magnitude of collective anti-predator responses This possibility has yet to be tested. Given the costs of joining a mobbing event and the potential benefits of selective responses, we predicted that the number of jackdaws responding to a scolding call would depend on the identity of the caller. Given the greater threat levels, we predicted less discriminating responses to playbacks within nestbox colonies

Methods

Results

Conclusion