The higher the better: Sentinel height influences foraging success in a social bird

School of Biological Sciences, University of Bristol, Woodland Road, Bristol BS8 1UG, UK.
Proceedings of the Royal Society B: Biological Sciences (Impact Factor: 5.05). 05/2009; 276(1666):2437-42. DOI: 10.1098/rspb.2009.0187
Source: PubMed


In all social species, information relevant to survival and reproduction can be obtained in two main ways: through personal interaction with the environment (i.e. 'personal' information) and from the performance of others (i.e. 'public' information). While public information is less costly to obtain than personal information, it may be inappropriate or inaccurate. When deciding how much to rely on public information, individuals should therefore assess its potential quality, but this possibility requires empirical testing in animals. Here, we use the sentinel system of cooperatively breeding pied babblers (Turdoides bicolor) to investigate how behavioural decisions of foragers are influenced by potential variation in the quality of anti-predator information from a vigilant groupmate. When sentinels moved to a higher position, from where their probability of detecting predators is likely to be greater, foragers reduced their vigilance, spread out more widely and were more likely to venture into the open. Consequently, they spent more time foraging and increased their foraging efficiency, resulting in a profound increase in biomass intake rate. The opposite behavioural changes, and consequent foraging outcomes, were found when sentinels moved lower. A playback experiment demonstrated that foragers can use vocal cues alone to assess sentinel height. This is the first study to link explicitly a measure of the potential quality of public information with a fitness measure from those relying on the information, and our results emphasize that a full understanding of the evolution of communication in complex societies requires consideration of the reliability of information.

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Available from: Andrew N Radford
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    • "Use of vocal cues, such as the watchman's song (Wickler, 1985), enables group members to coordinate vigilance more efficiently. In other species, the watchman's song is known to provide information about sentinel presence (Holl en et al., 2008; Manser, 1999) and height (Radford et al., 2009), as well as estimates of current risk level (Bell et al., 2009), and to be used in negotiation with foragers about bout duration (Bell et al., 2010). Dwarf mongoose sentinels are known to produce a watchman's song, although not during every bout (Kern & Radford, 2013), and foragers appear to use it "
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    ABSTRACT: To maximise survival, animals should adjust their behaviour flexibly in response to indicators of predation risk. Predation risk is affected by a range of ecological, social and individual variables, which can fluctuate over different time scales. In general, current risk levels are known to influence the behaviour of sentinels, individuals that adopt a raised position to scan for danger while group-mates are engaged in other activities. However, there has been little consideration of whether decisions made at different stages of a sentinel bout are affected in the same way by perceived predation risk and whether the same level of behavioural plasticity is exhibited when making these different decisions. Here we use detailed behavioural observations and a playback experiment to investigate the behavioural choices of dwarf mongoose (Helogale parvula) sentinels at three different stages of a bout (before, start, during). Individuals were more likely to begin a bout, and did so sooner, following alarm calls, which are immediate, direct indicators of elevated risk. Sentinels selected an initial height from which to guard depending on factors that tend to vary in the medium-term (hours), choosing higher positions in denser habitat and less windy conditions. In contrast, decisions about bout duration were made in relation to short-term (seconds/minutes) changes in information, with sentinels guarding for longer when an alarm call was given during a bout, and terminating bouts sooner when group-mates moved out of sight. Our results demonstrate that sentinel decisions are influenced by both direct and indirect indicators of likely predation risk and that sentinel behaviour is adjusted flexibly with regard to information presented on various time scales, highlighting the complexity of decision-making processes.
    Full-text · Article · Nov 2014 · Animal Behaviour
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    • "As Bednekoff (1997, 2001) pointed out, a sentinel may be in a safe place so long as it is still safer than an individual that is foraging in the absence of a sentinel. This is very difficult to determine owing to the behavioural adjustments that individuals make: in pied babblers, individuals forage much closer to cover in the absence of a sentinel, reducing their risk of exposure to predators (Radford et al. 2009), but also reducing their foraging efficiency because their access to food patches is more limited (Hollén et al. 2008). Owing to these behavioural changes, individuals foraging in the absence of a sentinel may not necessarily be less safe, but instead suffer a lower energetic intake. "
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    ABSTRACT: Sentinel behaviour, where individuals take turns to watch for danger and give alarm calls to approaching predators, has been observed in a number of animal societies. However, the evolutionary causes of this behaviour remain unclear. There are two main, competing hypotheses regarding the evolution of sentinel behaviour. The first hypothesis is that it is a cooperative behaviour, where group members benefit from the detection of danger but share the workload of acting as a sentinel. The second is that it is a safe, selfish behaviour. Under the second hypothesis, once an individual is satiated, being a sentinel is safer because sentinels can detect threats more readily and can therefore escape from predators faster. We examined whether sentinels are safer than foragers in a wild, free-living cooperative bird (the pied babbler, Turdoides bicolor) with a well-described sentinel system. We found that sentinel behaviour was costly because (1) sentinels were targeted by predators more often, (2) they were further from cover than foragers, and (3) they took longer to reach the safety of cover following a predator alarm. These results suggest that individuals do not become sentinels because it is safer. This is the first study to demonstrate that sentinels are at greater risk of predator attack than foraging group members and suggests sentinel activity may have evolved as a form of cooperative behaviour.
    Full-text · Article · Jan 2013 · Animal Behaviour
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    • "For example, the low-amplitude 'chuck' calls produced by foraging individuals can mediate spacing between potential competitors (Radford & Ridley, 2008), provide information about group size and individual position and, thus, the need for personal vigilance (Radford & Ridley, 2007), indicate the current nutritional state of the forager (Radford & Ridley, 2008; Bell et al., 2010) and form the basis of negotiation over sentinel duty (Bell et al., 2010). Likewise, the 'watchman's song' of sentinels provides information about their presence (Hollén et al., 2008), height (Radford et al., 2009) and current level of risk in the area (Bell et al., 2009), as well as playing a role in negotiating this cooperative activity (Bell et al., 2010). These calls produced by individuals alone often vary in their structure and rate (see, for example, Radford & Ridley, 2008; Bell et al., 2009) and their usage can differ between group members of different sex and status (see, for example, Radford & Ridley, 2006). "
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    ABSTRACT: Individuals of many species communicate with one another using a range of vocalisations and there is often variation in the use and structure of these calls depending on sex, status and context. In social species, two or more group members may also combine their vocalisations to produce duets or choruses. While the function of duets and the different contributions of males and females have received considerable research attention, less is known about the different calls used by group members in choruses. Southern pied babblers (Turdoides bicolor) are cooperatively breeding birds of Southern Africa that live in permanent stable groups. In addition to a variety of calls given by individuals, group members frequently combine to produce raucous choruses which include several different call types. Here we describe these different call types for the first time and explore their usage, with respect to the sex and dominance status of callers, production of the call alone or as part of a chorus, and the social context. Eight out of nine possible calls used in choruses on some occasions were found to be statistically distinct. As expected from the variation shown in individual calls, some of those call types included in choruses were sex-specific and some were used more by dominants than subordinates. Moreover, there was variation in the use of different call types as solos and within choruses, as well as their occurrence in different contexts. We discuss what might be concluded about the functions of the different call types from their patterns of usage.
    Full-text · Article · Jan 2013 · Behaviour
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