Browsing by Author "Cook, Timothée R"

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    Buoyancy under control: underwater locomotor performance in a deep diving seabird suggests respiratory strategies for reducing foraging effort
    (Public Library of Science, 2010) Cook, Timothée R; Kato, Akiko; Tanaka, Hideji; Ropert-Coudert, Yan; Bost, Charles-André
    BACKGROUND: Because they have air stored in many body compartments, diving seabirds are expected to exhibit efficient behavioural strategies for reducing costs related to buoyancy control. We study the underwater locomotor activity of a deep-diving species from the Cormorant family (Kerguelen shag) and report locomotor adjustments to the change of buoyancy with depth. METHODOLOGY/PRINCIPAL FINDINGS: Using accelerometers, we show that during both the descent and ascent phases of dives, shags modelled their acceleration and stroking activity on the natural variation of buoyancy with depth. For example, during the descent phase, birds increased swim speed with depth. But in parallel, and with a decay constant similar to the one in the equation explaining the decrease of buoyancy with depth, they decreased foot-stroke frequency exponentially, a behaviour that enables birds to reduce oxygen consumption. During ascent, birds also reduced locomotor cost by ascending passively. We considered the depth at which they started gliding as a proxy to their depth of neutral buoyancy. This depth increased with maximum dive depth. As an explanation for this, we propose that shags adjust their buoyancy to depth by varying the amount of respiratory air they dive with. Conclusions/Significance Calculations based on known values of stored body oxygen volumes and on deep-diving metabolic rates in avian divers suggest that the variations of volume of respiratory oxygen associated with a respiration mediated buoyancy control only influence aerobic dive duration moderately. Therefore, we propose that an advantage in cormorants - as in other families of diving seabirds - of respiratory air volume adjustment upon diving could be related less to increasing time of submergence, through an increased volume of body oxygen stores, than to reducing the locomotor costs of buoyancy control.
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    Can foraging ecology drive the evolution of body size in a diving endotherm?
    (Public Library of Science, 2013) Cook, Timothée R; Lescroël, Amélie; Cherel, Yves; Kato, Akiko; Bost, Charles-André
    Within a single animal species, different morphs can allow for differential exploitation of foraging niches between populations, while sexual size dimorphism can provide each sex with access to different resources. Despite being potentially important agents of evolution, resource polymorphisms, and the way they operate in wild populations, remain poorly understood. In this study, we examine how trophic factors can select for different body sizes between populations and sexes in a diving endotherm. Dive depth and duration are positively related to body size in diving birds and mammals, a relationship explained by a lower mass-specific metabolic rate and greater oxygen stores in larger individuals. Based on this allometry, we predict that selection for exploiting resources situated at different depths can drive the evolution of body size in species of diving endotherms at the population and sexual level. To test this prediction, we studied the foraging ecology of Blue-eyed Shags, a group of cormorants with male-biased sexual size dimorphism from across the Southern Ocean. We found that mean body mass and relative difference in body mass between sexes varied by up to 77% and 107% between neighbouring colonies, respectively. Birds from colonies with larger individuals dived deeper than birds from colonies with smaller individuals, when accounting for sex. In parallel, males dived further offshore and deeper than females and the sexual difference in dive depth reflected the level of sexual size dimorphism at each colony. We argue that body size in this group of birds is under intense selection for diving to depths of profitable benthic prey patches and that, locally, sexual niche divergence selection can exaggerate the sexual size dimorphism of Blue-eyed Shags initially set up by sexual selection. Our findings suggest that trophic resources can select for important geographic micro-variability in body size between populations and sexes.
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    Investigating the foraging ecology and energy requirements of a seabird population increasing in an intensely exploited marine environment
    (2017) Gaglio, Davide; Ryan, Peter G; Cook, Timothée R; Sherley, Richard B
    Their high energetic demands make seabirds sensitive to changes in prey availability, which is often reflected in their diet and energetic expenditure during breeding. Populations of the three seabirds endemic to southern Africa's Benguela upwelling ecosystem that rely on small pelagic fish have decreased dramatically over the last decade. In contrast, the population of the greater crested tern Thalasseus bergii has increased. To understand these conflicting trends, I investigated the foraging ecology and energy requirements of greater crested tern breeding in the Western Cape, South Africa. Diet was assessed by a novel non-invasive methodology developed in this study, using digital photography. More than 24,000 prey items from at least 51 different prey taxa were identified, with 34 new prey species recorded, revealing a high degree of foraging plasticity for this seabird. Greater crested terns rely mainly on anchovy Engraulis encrasicolus (65%), which averaged 84 mm long. Prey composition differed significantly between breeding stages, with anchovy especially dominant at the onset of the breeding period and the diet becoming more variable as the season progressed. Time-energy models for breeding terns were built based on activity budgets collected from non-invasive video-recordings and focal observations. Foraging trips were significantly longer during incubation than the chick provisioning stages, and feeding rates doubled from early to late chick provisioning. This study illustrated a steady increase in energy needs of adults throughout the breeding season, due to their increased foraging effort to meet chick energy needs. In comparison to other Benguela endemic seabirds that also rely on small pelagic fish, terns displayed substantially lower energy requirements at both individual and population levels. I also explored the benefits underlying interactions within mixed-species aggregations by investigating the costs induced by kleptoparasitism between mixed colonies of greater crested terns and Hartlaub's gulls Chroicocephalus hartlaubii and colonies with greater crested terns alone. Video-recordings coupled with focal observations showed that terns suffer direct costs to chick provisioning rates and indirect costs through energy expenditure in a mixed-species colony, suggesting that these breeding assemblages may be a form of parasitism rather than a mutualistic association. Despite the detrimental effects of interspecific kleptoparasitism, the marked foraging plasticity and low energetic requirements of greater crested terns, described in this study, coupled with specific life history traits such as low fidelity to breeding sites and extended post-fledging care, are key factors that allow this species to cope with changes in the availability and abundance of their main prey. Understanding species-specific behavioural responses to ecosystem variations in the Benguela upwelling system is vital for assessing the impact of commercial fisheries on seabird populations and fish stocks. Finally, the implementation of the method developed in this study, in long-term monitoring programmes, may provide crucial knowledge for conservation plans and key input to realising an ecosystem approach to fisheries management.