Browsing by Author "Ridley, Amanda R"

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    Calling by concluding sentinels: coordinating cooperation or revealing risk?
    (Public Library of Science, 2011) Hollén, Linda I; Bell, Matthew B V; Russell, Alexis; Niven, Fraser; Ridley, Amanda R; Radford, Andrew N
    Efficient cooperation requires effective coordination of individual contributions to the cooperative behaviour. Most social birds and mammals involved in cooperation produce a range of vocalisations, which may be important in regulating both individual contributions and the combined group effort. Here we investigate the role of a specific call in regulating cooperative sentinel behaviour in pied babblers ( Turdoides bicolor ). ‘Fast-rate chuck’ calls are often given by sentinels as they finish guard bouts and may potentially coordinate the rotation of individuals as sentinels, minimising time without a sentinel, or may signal the presence or absence of predators, regulating the onset of the subsequent sentinel bout. We ask (i) when fast-rate chuck calls are given and (ii) what effect they have on the interval between sentinel bouts. Contrary to expectation, we find little evidence that these calls are involved in regulating the pied babbler sentinel system: observations revealed that their utterance is influenced only marginally by wind conditions and not at all by habitat, while observations and experimental playback showed that the giving of these calls has no effect on inter-bout interval. We conclude that pied babblers do not seem to call at the end of a sentinel bout to maximise the efficiency of this cooperative act, but may use vocalisations at this stage to influence more individually driven behaviours.
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    Calling where it counts: subordinate pied babblers target the audience of their vocal advertisements
    (Public Library of Science, 2015) Humphries, David J; Finch, Fiona M; Bell, Matthew B V; Ridley, Amanda R
    For territorial group-living species, opportunities to reproduce on the natal territory can be limited by a number of factors including the availability of resources within a territory, access to unrelated individuals, and monopolies on reproduction by dominant group members. Individuals looking to reproduce are therefore faced with the options of either waiting for a breeding opportunity to arise in the natal territory, or searching for reproductive opportunities in non-natal groups. In the cooperatively breeding Southern pied babbler, Turdoides bicolor , most individuals who achieve reproductive success do so through taking up dominant breeding positions within non-natal groups. For subordinate pied babblers therefore, searching for breeding opportunities in non-natal groups is of primary importance as this represents the major route to reproductive success. However, prospecting (where individuals leave the group to search for reproductive opportunities within other groups) is costly and individuals rapidly lose weight when not part of a group. Here we demonstrate that subordinate pied babblers adopt an alternative strategy for mate attraction by vocal advertisement from within their natal territories. We show that subordinates focus their calling efforts on the edges of their territory, and specifically near boundaries with neighbouring groups that have potential breeding partners (unrelated individuals of the opposite sex). In contrast to prospecting, calling individuals showed no body mass loss associated with this behaviour, suggesting that calling from within the group may provide a ‘cheap’ advertisement strategy. Additionally, we show that subordinates use information regarding the composition of neighbouring groups to target the greatest number of potential mating partners.
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    Can sociality buffer the impacts of climate change on a cooperatively-breeding bird, the southern pied babbler Turdoides bicolor?
    (University of Cape Town, 2020) Bourne, Amanda R; Cunningham, Susan J; Ridley, Amanda R; Spottiswoode, Claire
    Increasingly harsh and unpredictable climate regimes are affecting animal populations everywhere and understanding how species respond to current environmental variability is important for predicting vulnerability to climate change over longer timescales. Species living in characteristically harsh and unpredictable arid and semi-arid ecosystems are useful models for studying impacts of climate variability and change because these ecosystems are experiencing rapid increases in both average and maximum temperatures, as well as increased interannual rainfall variation, as a result of anthropogenic climate change. That animals living in highly variable environments are disproportionately more likely to engage in cooperative breeding implies that this strategy may buffer individuals against the negative effects of adverse climate conditions. An aspect of species' vulnerability to climate change that remains relatively unexplored is whether responses to environmental stressors might therefore be mitigated by sociality, particularly in those species in which group members are highly cooperative. In this thesis, I use behaviour, morphology, and physiology data that I collected over three consecutive austral summer field seasons (2016-2019) and A. Prof. Amanda Ridley's 15-year life history dataset (2003-2019, to which I contributed the last three years of data) for a cooperatively-breeding bird, the southern pied babbler Turdoides bicolor. I investigate the impacts of temperature, rainfall, and group size on interannual survival, behaviour, physiology, growth, and reproduction in southern pied babblers, taking a multidisciplinary approach combining behavioural ecology, life history, and ecophysiology. In order to avoid disturbance to the study population, I validated and implemented a non-invasive method for collecting physiological measurements (daily energy expenditure and water turnover). I also tested for the influence of interactions between weather and group size variables because the presence of significant interactions would provide evidence in support of a moderating effect of sociality. I found that exposure to high temperatures significantly constrained successful breeding and the interannual survival of both breeding adults and juvenile birds, and explored the mechanisms behind these observed relationships: adjustments in parental care behaviour, body mass loss, reduced nestling growth rates, and the physiological costs of care at high temperatures. Higher rainfall and larger groups sizes were generally associated with higher reproductive success and survival, but I found no evidence for an interaction between weather variables and group size: individuals across all group sizes experienced similar effects of conditions. I therefore conclude that 1) pied babblers will increasingly face challenges for population recovery and persistence in the near future as survival and reproduction are increasingly compromised by ever higher temperatures, and 2) a life history strategy that relies on the presence of helpers for successful breeding is unlikely to buffer individual group members against climatic variability and climate change in this cooperatively breeding species.