STABLE NITROGEN AND CARBON ISOTOPES MAY NOT BE GOOD INDICATORS OF ALTITUDINAL DISTRIBUTIONS OF MONTANE PASSERINES

Abstract We examined δ15N and δ13C values of feathers from nine species, belonging to three feeding guilds (herbivores, omnivores, and insectivores), of wild passerines at eight sites along an altitudinal gradient (339–2,876 m asl) within Taroko National Park, Taiwan. We examined: (1) if altitudinal patterns in feather δ15N and δ13C are consistent with previously published values for plants and soils, (2) if feather δ15N and δ13C differ among sites, and (3) if there are year-to-year and/or month-to-month fluctuations in feather δ15N and δ13C of the same birds. We found no simple relationship between feather isotope values and altitude. Feather δ15N values decreased significantly with increasing altitude for insectivores, but not for herbivores and omnivores. Feather δ13C values increased significantly with increasing altitude for herbivores and omnivores, but not for insectivores. Altitudinal trends in feather δ15N and δ13C values exhibit even more inconsistent patterns when data were analyzed by species;...