Using stable isotopes to investigate the dietary trophic level of fledgling songbirds

Use of early successional habitat by mature forest birds during the postfledging period is well documented, but reasons for this habitat shift remain elusive. Although forest-breeding songbirds are primarily insectivorous during the nestling and early fledgling periods due to high protein requirements, older fledglings may adopt a heavily frugivorous diet. Our objectives were to use stable isotopes to examine the dietary trophic level of juveniles of three species of mature forest songbirds to determine if juvenile songbirds heavily consume fruit resources during the postfledging period and to evaluate a possible link between diet and energetic condition. We collected the outer right rectrix and several body feathers from 34 Wood Thrushes (Hylocichla mustelina), 34 Ovenbirds (Seiurus aurocapilla), and 35 Scarlet Tanagers (Piranga olivacea) captured in regenerating clearcuts in southeastern Ohio in 2005 and 2006. We also collected fruit and arthropod samples from each clearcut. Isotopic values of body feathers were significantly higher (more enriched) than those of rectrices in all cases except values of d13C for Ovenbirds where we found no difference between body and rectrix feathers. These results suggest that juvenile songbirds did not undergo a strong shift to frugivory during the postfledging period, and arthropods were the primary source of protein during the period when rectrix and body feathers were growing. In addition, the energetic condition of birds was not related to the isotopic signature of feathers. Although our results are inconsistent with the hypothesis that juveniles move into regenerating clearcuts enabling them to shift to a primarily frugivorous diet during the postfledging period, they may consume fruit for nonprotein requirements, such as lipids and carbohydrates.