Using stable isotopes to assess seasonal patterns of avian predation across a terrestrial-marine landscape

In this study, we used the stable isotope ratios N-15/N-14 and C-13/C-12 to clarify the spatial and temporal patterns of small mammal derpredation by wide-ranging raptors across a salt marsh and grassland landscape. To determ ine whether clear isotope signals existed for the two adjoining habitats, a nd if these differences could allow for an assessment of the seasonal habit at origins of depredated rodents, primarily the vole, Microtus californicus , we investigated the nitrogen and carbon isotope ratios for three sets of data. First, to establish potential differences in isotope signatures betwe en the two habitats, we collected plant species consumed by the vole. Secon d, we analyzed bone collagen from voles originating from known locations al ong a gradient from marsh to grassland. Finally, the signatures of these un predated voles were then used to categorize isotope values of vole remains found in the pellets of their raptor predators. Results indicated that mean delta N-15 and delta C-13 were both higher in marsh plants than in grassla nd species. Although both isotopes showed atrophic level increase from plan t to bone collagen, nitrogen provided the more informative signature when a ttempting to distinguish the habitat origins of voles. We found that the de lta N-15 values of unpredated voles were significantly related to the dista nce of the specimen along the marsh-grassland interface, with higher values in the marsh habitat. Analysis of the depredated bones found in raptor pel lets indicated that a greater percentage of voles originated in grassland d uring the spring/summer season (88%), as compared with the fall/winter peri od (60%). Finally, we compared this pattern of habitat and season-specific predation to another measure of predation risk, the relative encounter rate s of raptors foraging for voles, and found that the two estimates were simi lar except during the winter season. This study demonstrates the usefulness of stable isotope analysis for elucidating seasonal patterns of predation across a complex landscape.