The cessation of contact calls does not provoke or modulate alarm behaviour in a social passerine

Abstract

Vocalizations that signal predation risk such as alarm calls provide crucial information for the survival of group-living individuals. However, alarm calling may attract the predator’s attention and, to avoid this cost, animals can opt for alternative strategies to indicate danger, such as ‘adaptive silence’, which is the cessation of vocalizations. We investigate here whether abrupt contact call cessation would provoke alarm responses, or would reinforce the signal given by an alarm call. In an aviary setting, we conducted playback experiments with a group-living passerine, the Swinhoe’s white-eye, Zosterops simplex. We found that birds did not respond to a sudden call cessation, nor did they have a stronger response to alarm calls followed by silence than to alarm calls followed by contact calls. Confirming previous work investigating contact call rate, it appears that in this species contact calls encode information about social factors but not environmental conditions.

Audience effects in a group‐living bird: How contact call rate is affected by vegetation and group size and composition

“…Rather, social stimuli, and specifically the presence of affiliated individuals and individuals of similar levels on the dominance hierarchy, were associated with higher individual‐level calling rates. Together with other studies on Swinhoe's White‐eyes (Meaux, He, et al, 2021; Meaux, Peabotuwage, et al, 2021), these findings suggest that changes in contact call rate in this species are more influenced by social context than by environmental conditions. Below, we discuss in detail the effects of vegetation density, and then the effects of group size and group composition on the different call types.…”

“…While we recognized that sudden silence might not be easy to attain without all group members detecting a threat, we thought that silence could reinforce the reliability of an alarm signal and that contact calls resuming after an alarm call would indicate that it was a false alarm (Goodale & Kotagama, 2005; Munn, 1986; Wolf et al, 2013). However, an experimental study performed simultaneously to this one demonstrated that group‐level contact call rate did not appear to be related to predation risk (Meaux, He, et al, 2021). Indeed, after exposure to a predator model, the birds did not abruptly cease calling.…”

“…Once a group was placed in captivity, it was observed for 5-8 days during a pre-experimental, habituation period. After this, the group was subjected to the group size and vegetation experiments, as well as other simultaneous experiments discussed elsewhere (Meaux, He, et al, 2021), for on average seven additional days, before being released at the capture site. Sometimes wild white-eyes were present in the area, but because of the lack of trees near the aviary, they did not get within 10 m of it.…”

“…We also noticed that white‐eyes have a strong dominance hierarchy, notably when competing for access to a feeder, and predicted that calling rate would be lower when birds of opposite dominance status were present together, specifically due to subordinates withholding information about their location (Wiley, 1983). Finally, like other contact call systems (Ey & Fischer, 2011; Krama et al, 2008; Krams, 2001), white‐eyes use distinct contact call types, including soft “short‐distance” and louder “long‐distance” calls, as well as a “flight call,” which is only produced by a flying individual (Meaux, He, et al, 2021; Robertson, 1996). We therefore expected different contact call types would show different relationships with group size, predicting at the individual‐level: (a) an increase in short‐distance calls in larger groups when there are more individuals nearby the signaller; (b) a high production of long‐distance calls at reduced group sizes when individuals may signal that they have become isolated from the group; (c) an increase in flight calls in larger groups within which individuals may fly more frequently due to higher competition and/or aggression.…”

“…We also noticed that white-eyes have a strong dominance hierarchy, notably when competing for access to a feeder, and predicted that calling rate would be lower when birds of opposite dominance status were present together, specifically due to subordinates withholding information about their location (Wiley, 1983). Finally, like other contact call systems (Ey & Fischer, 2011;Krama et al, 2008;Krams, 2001), white-eyes use distinct contact call types, including soft "short-distance" and louder "long-distance" calls, as well as a "flight call," which is only produced by a flying individual (Meaux, He, et al, 2021;Robertson, 1996). We therefore expected different contact call types would show different relationships with group size, predicting at the individual-level:…”