Physiological and environmental factors shape foraging strategies and energy balance. For species that breed seasonally, physiological changes in an individual can have short-term effects, but also can persist as carry-over effects from one season to the next, such as from the overwintering season to the breeding season. We tested the hypothesis that reproductive performance could be predicted by diet and energy balance during the breeding and nonbreeding seasons in a long-lived seabird, the Leach's Storm-Petrel (Oceanodroma leucorhoa). Specifically, we predicted that better reproductive performance would be correlated with four factors: (1) a high-lipid diet, as indexed by a high C:N ratio in stable isotope analyses; (2) a diet rich in antioxidants, as indexed by high plasma antioxidant capacity; (3) foraging at a high trophic level, as indexed by high values of δ15N in stable isotope analyses, which is positively related to lipids; and (4) a positive long-term energy balance, revealed by low levels of corticosterone in feathers. Because of our interest in short-term effects vs. carry-over effects, stable isotope values were measured from two different tissue sources: erythrocytes, to test for short-term effects, and winter-grown feathers, to test for carry-over effects. We monitored reproductive performance through egg volume, chick growth, parental provisioning, and fledging success. Parents with more breeding experience were more likely to have a successful nest in 2010, but not in 2009. Individuals exhibited consistent egg volume and nonbreeding season feather δ15N values across the 2 years of our study, but, overall, neither diet nor feather corticosterone predicted reproductive performance. Nonetheless, our simple, noninvasive measures of breeding performance could be applied to other species to study life-history strategies and energy balance.
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Vol. 132 • No. 1