Abstract
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 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 der...
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