Hermit thrushes are representative of several species of songbirds overwintering in the Southeast that are presumed to rely on berries as their winter food source. The diets of hermit thrushes varied during their winter stay on Spring Island. Birds with access to more fruit, particularly beautyberry, tended to have smaller territories. Many thrushes depended on insects over the winter, especially towards spring. The birds that change their diet (fruit to insects) over the winter suffered reduced health.

The role of diet in limiting migratory birds during the non-breeding period is poorly understood in part due to the complexities of quantifying food availability and avian diet. We tracked overwinter changes in arthropod and fruit availability for the short-distance migratory bird, the Hermit Thrush (Catharus guttatus), in a mixed hardwood-pine forest the southeastern United States.

Fruit availability declined over winter and arthropod abundance declined with declining temperature. Concurrently, using fecal samples and stable isotopes, we tracked diet consumption. Fruit declined in fecal samples from early to mid season and ?¹³C and ?¹⁵N isotope signatures in blood became more enriched, consistent with a decline in fruit consumption and an increase in arthropod consumption. Larger-bodied birds, predominately males, maintained territories with higher arthropod abundance, and had a greater proportion of arthropods in their diet and less variation in ?13C and fat loads overwinter. In contrast, smaller bodied females gained fat midwinter, likely to cope with 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 non-breeding season food limitation and size and sex-mediated dominance behavior on both within season condition of birds and on subsequent breeding success and survival.