The role of food in limiting migratory birds during the nonbreeding period is poorly understood, in part because of the complexities of quantifying food availability and diet. We tracked overwinter changes in the availability of arthropods and fruits, the primary winter foods of the Hermit Thrush (Catharus guttatus), a short-distance migrant. Fruit availability declined over the winter, and arthropod availability fluctuated with changing temperature. Concurrently, using fecal samples and stable isotopes, we tracked relative food consumption. In fecal samples fruit declined from early to mid season and δ13C and δ15N isotope signatures in blood became more enriched, consistent with a decline in fruit consumption and an increase in arthropod consumption. Larger-bodied birds, predominantly males, maintained territories in which the abundance of arthropods was higher, had a greater proportion of arthropods in their diet and less variation in δ13C (indicator of a stable diet) and fat loads over the winter. In contrast, smaller-bodied birds, primarily females, gained fat midwinter in response to unpredictable and lower-quality resources. These results are consistent with both a size-mediated form of dominance and sexual habitat segregation, such that smaller bodied birds, mainly females, may be behaviorally excluded from optimal territories. Future research should focus on the long-term consequences of food limitation in the nonbreeding season and size and sex-mediated dominance behavior on both the condition of birds within a season and on subsequent breeding success and survival.
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Vol. 113 • No. 3