Combining Stable-Isotope and Body-Composition Analyses to Assess Nutrient-Allocation Strategies in Breeding White-Winged Scoters (Melanitta fusca)

Abstract

Birds meet the energy and nutrient demands of egg formation by using dietary (exogenous) sources, somatic (endogenous) nutrients, or combinations of both. Therefore, understanding plasticity in resource acquisition and the allocation strategies that are used is important for predicting how ecosystem changes across a species' range could affect vital rates. Sources of egg nutrients have traditionally been assessed through analyses of body composition, but stable-isotope analysis has provided a new tool in cases where animal tissues differ isotopically from the local food webs where they breed. We provide the first simultaneous comparison of these two techniques and test the "seasonally variable nutrient threshold hypothesis." Using body-composition analysis of Whitewinged Scoters {Melanitta fusca) collected at the northern extent of their range, we inferred that protein in rapidly developing ovarian follicles was derived entirely from dietary sources but that follicle lipids were most likely derived largely from somatic reserves. Stableisotope analysis confirmed that most protein was derived from dietary sources during early (70.7-86.6%) and late (83.4-94.4%) spring collection periods, but dietary lipids were not distinguishable isotopically from stored somatic lipids to estimate lipid contributions to eggs. This pattern dififered from that at the southern limit of the species' breeding range, where both lipid and protein for egg formation were derived from exogenous sources. Although our results were consistent with plasticity in resource-allocation strategies among sites (latitudinal variation), they did not support the seasonally variable nutrient threshold hypothesis within a site. We discuss the benefits of using both techniques simultaneously to assess reproductive strategies of birds that migrate between isotopically distinct areas. Une combinaison de l'analyse des isotopes stables et de celle de la composition corporelle pour évaluer les stratégies d'allocation des nutriments chez des individus reproducteurs de Melanitta fuscaRÉSUMÉ.-Les oiseaux comblent leurs besoins en énergie et en nutriments pour la formation des oeufs en utilisant des sources alimentaires (exogènes), des nutriments somatiques (endogènes) ou une combinaison des deux. Ainsi, comprendre la plasticité de l'acquisition des ressources et les stratégies d'allocation utilisées s'avère essentiel pour prédire la façon dont les changements écosystémiques à travers l'aire de répartition d'une espèce peuvent influer sur les paramètres démographiques. Les sources de nutriments pour les oeufs ont traditionnellement été évaluées par des analyses de la composition corporelle. L'analyse des isotopes stables constitue un nouvel outil pour les cas où les tissus des animaux diffèrent, sur le plan des isotopes, des chaînes alimentaires locales où ils se reproduisent. Nous fournissons la première comparaison simultanée de ces deux techniques et testons l'hypothèse d'un seuil des nutriments variable au cours de la saison. À l'aide d'une analyse de la composi...

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