Using stable isotopes to track frugivory in migratory passerines

Abstract

Several species of North American migratory songbirds undergo seasonal diet shifts from insects to fruits, but this phenomenon is poorly quantified. Measurement of naturally occurring stable isotopes of carbon (δ 13 C) and nitrogen (δ 15 N) are linked to sources of diets and trophic level, respectively. We used stable carbon (δ 13 C) and nitrogen (δ 15 N) isotope analyses of blood and claw tissues of 16 species of migratory songbirds to evaluate the timing and extent of frugivory over different periods. Species differed considerably in their tissue δ 15 N values, but we found poor isotopic segregation of species according to our a priori classifications as insectivores or omnivores. Season accounted for considerable variance in tissue δ 15 N values. However, only American Robin ( Turdus migratorius L., 1766), Northern Oriole ( Icterus galbula (L., 1758)), Gray Catbird ( Dumetella carolinensis (L., 1766)), Least Flycatcher ( Empidonax minimus (W.M. Baird and S.F. Baird, 1843)), and Warbling Vireo ( Vireo gilvus (Vieillot, 1808)) showed expected decrease in winter-grown tissue δ 15 N values compared with those grown in late summer. This indicates either that our a priori guild associations were incorrect and (or) that using stable isotopes to track frugivory at continental scales is problematic. We recommend that the isotope technique be used to track frugivory only in well-constrained systems where food-web δ 15 N follows reliable and understood trophic enrichment patterns.