Evidence for individual discrimination and numerical assessment in collective antipredator behaviour in wild jackdaws ( Corvus monedula )

Abstract

Collective responses to threats occur throughout the animal kingdom but little is known about the cognitive processes underpinning them. Antipredator mobbing is one such response. Approaching a predator may be highly risky, but the individual risk declines and the likelihood of repelling the predator increases in larger mobbing groups. The ability to appraise the number of conspecifics involved in a mobbing event could therefore facilitate strategic decisions about whether to join. Mobs are commonly initiated by recruitment calls, which may provide valuable information to guide decision-making. We tested whether the number of wild jackdaws responding to recruitment calls was influenced by the number of callers. As predicted, playbacks simulating three or five callers tended to recruit more individuals than playbacks of one caller. Recruitment also substantially increased if recruits themselves produced calls. These results suggest that jackdaws use individual vocal discrimination to assess the number of conspecifics involved in initiating mobbing events, and use this information to guide their responses. Our results show support for the use of numerical assessment in antipredator mobbing responses and highlight the need for a greater understanding of the cognitive processes involved in collective behaviour.

Mobbing responses of great tits (Parus major) do not depend on the number of heterospecific callers

“…Responses were higher during playbacks simulating multiple callers than during the playbacks simulating only one caller, indicating that great tits use numerical competence to assess whether they join a mobbing flock of conspecific individuals. Similar results have been found in only one other bird species (Coomes et al., 2019). Since being in a larger group dilutes the risk of death during a mobbing event (Foster & Treherne, 1981; Hamilton, 1971; Hogan et al., 2017) and large groups are more efficient than smaller groups at deterring predators (Krams et al., 2009; Robinson, 1985), the capacity to evaluate the number of mobbers (conspecifics and heterospecifics) before joining the mobbing flock could be highly beneficial for individuals to join.…”

Who is crying wolf? Seasonal effect on antipredator response to age-specific alarm calls in common ravens, Corvus corax

“…Moreover, the played-back individuals were unfamiliar to the tested ravens, indicating that receivers can extract age-class information from any conspecific alarm calls, which is perfectly in line with the ecological relevance of alarm calls (Gill & Bierema, 2013) and the structure of raven foraging groups with moderate to high fission-fusion dynamics (Braun, Walsdorff, Fraser, & Bugnyar, 2012). Contrary to our expectation and to recent findings in jackdaws (Coomes et al, 2019), we could not find any effect of the number of played-back individuals on ravens' antipredator behaviour. Our failure to detect a numerical discrimination through alarm calls may be due to the salience of the chosen stimuli.…”

“…Taken together, our study contributes to our understanding of what type of information birds may pick up when hearing alarm calls. While most studies on alarm calls have focused on functional reference about predators (Evans et al, 1993;Griesser, 2008;Suzuki, 2011Suzuki, , 2014, relatively few studies have experimentally tested for other types of information, like familiarity of caller/group membership (Griesser & Ekman, 2004Woods et al, 2018), number of callers and cal-lers´age class (Coomes et al, 2019; this study). The findings reveal that birds respond selectively to different features that appear to be ecologically relevant, like the seasonal effect of responding to adults in this study.…”

“…Recent experiments on jackdaws (Corvus monedula) revealed that birds respond stronger with collective anti-predator behaviour to the play back alarm calls of colony members as compared to noncolony members, indicating that receivers discriminate between familiar and unfamiliar birds (Woods, Kings, McIvor, & Thornton, 2018). Furthermore, the number of callers had a similar positive effect on the probability to engage in collective antipredator behaviour, indicating that receivers take into account whether the alarm calls were elicited by a single or a few birds and hence the intensity of the response (Coomes, Mcivor, Thornton, Coomes, & Thornton, 2019). Such assessment capability by receivers was also documented in small mammals, precisely in adult Richardson´s ground squirrels (Sloan & Hare, 2008).…”

“…In the present study, we followed the logic of the studies on jackdaws (Coomes et al, 2019) and investigated whether common ravens are attentive to sender-specific characteristics in alarm calls. Unlike jackdaws, adult ravens defend large territories (Scarpignato & George, 2011) and thus do not form colonies during breeding.…”