Savannah River NERP songbird tissue, insect, and fruit isotope database

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. 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. We measured carbon and nitrogen stable
isotope values in food items (fruits, C3 and C4 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. We combined these diet and tissue values with
empirically derived discrimination factors and used
concentration-dependent mixing models to quantify seasonal diet shifts. We
also validated mixing model results with data from fecal samples. 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. 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. Fecal data showed
qualitatively similar trends to mixing models, but consistently estimated
higher contributions of fruit. 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.