Stable isotope ratios of a tropical marine predator: confounding effects of nutritional status during growth
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Stable isotope analysis of carbon and nitrogen is frequently used to study the diets and foraging ecology of marine predators. However, isotopic values may also be affected by an individual’s nutritional status and associated physiological processes. Here, we use C and N stable isotopes in blood and feathers of blue-footed booby chicks at the Galápagos Islands to examine how isotopic values are related to body condition and growth rate, and to assess the consistency in the isotope ratios of individuals during growth. Size dimorphism in blue-footed boobies provided an additional opportunity to examine how isotope ratios differ between sexes in relation to body size and growth rate. There was no significant difference between sexes but both C and N stable isotopes were significantly negatively related to the body condition of chicks. These data were consistent with individual variation in physiological processes affecting fractionation, although we cannot rule out the possibility that they were also influenced to some extent by population-level variation in the stable isotope ratios of prey fed to chicks, for instance related to prey size, depth or lipid content. Our results highlight the need for methods that take proper account of confounding physiological factors in isotopic studies of foraging ecology and diet.
We thank Marilyn Cruz, Pamela Martínez, Sarah-L. Smith, Alberto Vélez and Pablo Mejía for their help in the field, and Virna Cedeño, Washington Tapia, David Vizuete, Miton Mora, Efraín García and Nelson García for logistical support and advice. We thank Natalia Tirado from the Charles Darwin Research Station for help identifying prey species. Stable isotope analyses were carried out at SUERC, UK, and we particularly thank Jason Newton for his help. We also thank two anonymous reviewers for their comments on previous versions of this manuscript. This study was funded by Consejo Nacional de Ciencia y Tecnología, Mexico, and carried out in collaboration with the Galápagos National Park Service and the Galápagos Genetics, Epidemiology and Pathology Laboratory with support from the UK government (DEFRA Darwin Initiative Grants 162-12-17 and EDIPO 15).
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