These methods have been used to successfully identify the species of birds that engage in frugivory, the kinds of fruit they consume, and the timing of fruit consumption. The most common methods for studying this phenomenon have included both direct and indirect observations of fruit consumption by birds and analysis of the contents of fecal samples. This shift is likely driven by reduced biomass of invertebrates during fall and winter and a concurrent increase in availability of fruits and seeds, which are generally conspicuous, energy rich, and easily “captured”. Our work indicates that fruits and seeds constitute substantial sources of sustenance for non-breeding songbirds, there is considerable separation of resource use among species in the fall and winter, and fecal estimates of contributions to songbird tissues should be interpreted cautiously.Ī relatively well-established example of a highly predictable diet shift is the seasonal transition by many species of passerine birds in temperate regions from invertebrate prey in the spring and summer to alternate resources (e.g., fruits and seeds) in the fall and winter. Fecal data showed qualitatively similar trends to mixing models, but consistently estimated higher contributions of fruit. Across this songbird community, estimated contributions of fruit to plasma/whole blood increased from 16.2 ± 7.5% in the fall (mean ± SD range: 4–26%) to 21.7 ± 10.3% (range: 9–37%) in the winter, while contributions of seeds increased from 29.4 ± 2.6% (range: 28–32%) in the fall to 36.6 ± 4.8% (range: 32–42%) in the winter. Diets in this bird community, as delineated N and C isotopic space, diverged in the fall and winter relative to the summer as consumption of fruits and seeds increased. We also validated mixing model results with data from fecal samples. We combined these diet and tissue values with empirically derived discrimination factors and used concentration-dependent mixing models to quantify seasonal diet shifts. We measured carbon and nitrogen stable isotope values in food items (fruits, C 3 and C 4 seeds, and insects from various trophic levels and plant communities) and in multiple tissues (feathers and plasma/whole blood) from 11 species of songbirds wintering in the southeastern U.S. However, relatively few studies have quantified diet composition or the extent of seasonal shifts during the non-breeding period, particularly among species and across communities with both residents and migrants. Many temperate passerine bird species switch from diets of mostly invertebrates in the spring and summer to diets that include fruit and seeds in the fall and winter.
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