Social rules govern vocal competition in the barn owl

Abstract

To resolve the share of limited resources, animals often compete through exchange of signals about their relative motivation to compete. When two competitors are similarly motivated, the resolution of conflicts may be achieved in the course of an interactive process. In barn owls, Tyto alba, in which siblings vocally compete during the prolonged absence of parents over access to the next delivered food item, we investigated what governs the decision to leave or enter a contest, and at which level. Siblings alternated periods during which one of the two individuals vocalized more than the other. Individuals followed turn-taking rules to interrupt each other and momentarily dominate the vocal competition. These social rules were weakly sensitive to hunger level and age hierarchy. Hence, the investment in a conflict is determined not only by need and resource-holding potential, but also by social interactions. The use of turn-taking rules governing individual vocal investment has rarely been shown in a competitive context. We hypothesized that these rules would allow individuals to remain alert to one another's motivation while maintaining the cost of vocalizing at the lowest level.© 2015 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd.Natural selection has favoured the evolution of behaviours and weapons to outcompete conspecifics, or of communication systems to resolve the share of resources (Maynard Smith, 1982;Parker, 1974). The term 'negotiation' is usually used for humans who bargain for resources and the process typically ends with a decision about which part of the resource each participant obtains (Nash, 1950). Evolutionary ecologists also use this concept to define situations in which animals communicate to reach an agreement about how a resource is shared or how to invest in a collaborative task (Johnstone & Hinde, 2006;Johnstone & Roulin, 2003;McNamara, Gasson, & Houston, 1999;Patricelli, Krakauer, & McElreath, 2011;Sirot, 2012). An individual that presents conspicuous ornaments or signals at higher levels than its opponents (e.g. produces louder begging calls in nestling birds) usually gains easier access to these limited resources (Godfray, 1991;Kilner, Noble, & Davies, 1999), but, for transient signals, this average signal level can vary over short periods of time, independently of variation in need or condition (Briffa, Elwood, & Dick, 1998;Greenfield, Tourtellot, & Snedden, 1997). The contest outcome is then the result of an interactive process settled during repeated interactions (Briffa et al., 1998;Enquist & Leimar, 1983;Enquist, Leimar, Ljungberg, Mallner, & Segerdahl, 1990;Payne & Pagel, 1996). These variations in signal level during competitive interactions raise the possibility that animals assess the temporal dynamics of signal production and not only the absolute signalling level of conspecifics to adjust their behaviour (Patricelli, Uy, Walsh, & Borgia, 2002). Game theory has dominated the way evolutionary biologists envisage social interactions (Dobler & Koll...

No need to shout: Effect of signal loudness on sibling communication in barn owls Tyto alba

“…Extensive variation in playback loudness did not significantly influence the investment of barn owl nestlings in sibling competition, contrary to variation in call rate and duration (present study and Ruppli et al, 2013;Dreiss, Ruppli, Faller, et al, 2015). Call loudness had weak or no influence on the negotiation process, as the playback loudness treatment did not affect the key acoustic parameters (call rate and duration) used in sibling negotiation.…”

“…When owlets are alone in the nest waiting for a parent to come back with a prey item, they continuously exchange hissing calls to advertise their hunger levels to siblings. Hungry individuals produce more calls and longer calls, which progressively deters their siblings from calling and competing for the food item next delivered by the parent (Dreiss, Ruppli, Faller, & Roulin, ; Ruppli, Dreiss, & Roulin, ). This communication system reduces the cost of sibling competition between related individuals (Johnstone & Roulin, ).…”

“…To reduce predation risk, nestlings may therefore be selected to produce informative vocal signals without having to call very loudly (Briskie, Martin, & Martin, 1999). More specifically, nestling barn owls follow specific turn-taking rules (Dreiss, Ruppli, Faller, et al, 2015) allowing them to avoid calling simultaneously (Dreiss, Ruppli, Oberli, et al, 2013). Hence, they would not need to call very loudly to overcome siblings.…”

Vocal communication regulates sibling competition over food stock

“…Roulin et al (2000) first used the term of Bsibling negotiation^to describe the vocal interaction between siblings preceding a parental feeding event. In sibling negotiation, nestlings vocally interact in the absence of parents Dreiss et al 2015), which influences the outcome of the pending competition (Dreiss et al 2010b). Previous empirical work and theory concluded that sibling negotiation Bis more likely to prove stable when the food provided by parents is nondivisible^ (Johnstone and Roulin 2003).…”

“…For each individual, we broadcast 14 times in a row the same 45-min playback sequence, each sequence followed by 15 min of silence. Owlets react in a similar way in front of unfamiliar playback calls and live nestlings' calls (they avoid overlapping playback calls, Dreiss et al 2013b; they refrain from calling in front of a highly vocal playback, Ruppli et al 2013a; they insert their calls between playback calls in the same way they do with live nestlings, Dreiss et al 2015) and are not physiologically stressed in nest boxes placed in the laboratory (Dreiss et al 2010a). …”