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; feather δ¹⁵N and δ¹³C values for some species increased significantly with increasing altitude, others decreased significantly with increasing altitude, and still others exhibited no significant relationship between isotopic values and altitude. The R² for the relationship between feather δ¹⁵N, δ¹³C values and altitude was generally low regardless of whether the analysis was by feeding guilds or species. This indicates much of the variation cannot be explained by altitude. There were either no significant differences between sites, or significant differences between some but not all sites when investigating δ¹⁵N or δ¹³C, whether by feeding guilds or by species. Our study suggests that carbon and nitrogen isotopes may be not useful markers to track altitudinal migration of montane passerines.