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1 January 2011 Combining Stable-Isotope and Body-Composition Analysesto Assess Nutrient-Allocation Strategies in BreedingWhite-Winged Scoters (Melanltta fusca)
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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 White-winged 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. Stable-isotope 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 differed 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.

© 2011 by The American Ornithologists' Union. All rights reserved. Please direct all requests for permission to photocopy or reproduce article content through the University of California Press's Rights and Permissions website,
Jean-Michel Devink, Stuart M. Slattery, Robert G. Clark, Ray T. Alisauskas, and Keith A. Hobson "Combining Stable-Isotope and Body-Composition Analysesto Assess Nutrient-Allocation Strategies in BreedingWhite-Winged Scoters (Melanltta fusca)," The Auk 128(1), 166-174, (1 January 2011).
Received: 16 March 2010; Accepted: 1 September 2010; Published: 1 January 2011

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