Efficiency and Significance of Multiple Vocal Signals in Sibling Competition

Abstract

Animals can compete for resources by displaying various acoustic signals that may differentially affect the outcome of competition. We propose the hypothesis that the most efficient signal to deter opponents should be the one that most honestly reveals motivation to compete. We tested this hypothesis in the barn owl (Tyto alba) in which nestlings produce more calls of longer duration than siblings to compete for priority access to the indivisible prey item their parents will deliver next. Because nestlings increase call rate to a larger extent than call duration when they become hungrier, call rate would signal more accurately hunger level. This leads us to propose three predictions. First, a high number of calls should be more efficient in deterring siblings to compete than long calls. Second, the rate at which an individual calls should be more sensitive to variation in the intensity of the sibling vocal competition than the duration of its calls. Third, call rate should influence competitors' vocalization for a longer period of time than call duration. To test these three predictions we performed playback experiments by broadcasting to singleton nestlings calls of varying durations and at different rates. According to the first prediction, singleton nestlings became less vocal to a larger extent when we broadcasted more calls compared to longer calls. In line with the second prediction, nestlings reduced vocalization rate to a larger extent than call duration when we broadcasted more or longer calls. Finally, call rate had a longer influence on opponent's vocal behavior than call duration. Young animals thus actively and differentially use multiple signaling components to compete with their siblings over parental resources.

Social rules govern vocal competition in the barn owl

“…During this long vocal interaction, nestlings avoid overlapping calls (Dreiss, Ruppli, Oberli, et al, 2013) and refrain from calling when facing a highly vocal competitor (Ruppli et al, 2013). These findings were all based on the analyses of average signalling levels, whereas in the present study we considered each single call to tackle the temporal Vocal dominance of the focal nestling in the exchange is the proportion of calls this nestling produced in the preceding sequence of 10 calls.…”

“…Eight playback sequences (constituting all the combinations of the three parameters) were broadcast to each nestling. Playback sequences were composed of 20 calls either longer than the mean call duration usually observed in vocal exchanges between two siblings (0.82e1.00 s, Ruppli et al, 2013) or shorter (0.60e0.78 s), of either continuously increasing duration (0.60e0.78 s or 0.82e1.00 s changing by 0.02 s every two calls) or continuously decreasing duration (0.78e0.60 s or 1.00e0.82 s changing by 0.02 s every two calls) and separated by either continuously increasing or decreasing pause duration (time interval between two calls starts from 3.05 to 9.94 s, changing by 0.76 s every two pauses). The eight playback sequences were broadcast in a random order and separated by a silence of 6 min.…”

“…Average and variation values were calculated on the last five calls of the focal individual and its sibling (for vocal exchange), and only if the last five calls were produced during the previous 2 min. This time lapse was chosen because the influence of playback calls on nestlings fades after a few minutes (Ruppli et al, 2013). However, results were qualitatively similar if we modified the criteria (previous 1 or 3 min and last three calls, not shown).…”

“…While parents are hunting, nestlings vocally compete and the most vocal nestling in the absence of parents has a higher chance of being fed when a parent returns than its less voluble siblings (Dreiss, Lahlah, & Roulin, 2010;Roulin, 2002). Each nestling invests in a sibesib vocal contest according to its hunger level, hungrier individuals producing longer calls at higher rates on average (Johnstone & Roulin, 2003, Roulin, 2002Ruppli, Dreiss, & Roulin, 2013), but also according to its siblings' vocalizations, withdrawing when its siblings produce longer calls at higher rates (Roulin, 2002;Ruppli et al, 2013). Such vocal competition during this so-called 'sibling negotiation' has been suggested in several taxa (Bulmer, Celis, & Gil, 2008;Madden, Kunc, English, Manser, & Clutton-Brock, 2009) and would limit the cost of food competition (Johnstone & Roulin, 2003), because individuals invest in competition according to their chance of winning.…”

Vocal communication regulates sibling competition over food stock

“…To build the playback sequences, we used calls of six owlets that did not consume any food in the preceding 28 h (aged 25-35 days, four males and two females); these owlets were recorded at night in the absence of parents in 2008 (see Ruppli et al 2013a). For each focal nestling, we built two unique playback sequences (one competitive and noncompetitive) using 20 different calls emitted by a single owlet.…”