UC Santa Cruz

Stable-isotope ratios of carbon (13C/12C; δ13C) and nitrogen (15N/14N; δ15N) in animal tissues are analyzed to estimate animal foraging ecology because these ratios reflect those of an animal's diet. This reflection is generally indirect, as stable-isotope ratios change with trophic level. These differences, called discrimination factors (reported as Δ), vary considerably by species and tissue. Variations in discrimination factors used in stableisotope mixing models can lead to inaccurate estimates of diets. Therefore, determining accurate discrimination factors specific to species and tissue is important. We established the Δ13C and Δ15N values between diet and blood and feathers from chicks and juveniles of the California Condor (Gymnogyps californianus). Hatchlings were fed rats for 76-119 days, whereas juveniles were fed dairy calves for 64 days. The mean Δ13C and Δ15N values ( ± SD) between chick feathers and rat muscle were 0.4 ± 0.4‰ and 3.1 ± 0.2‰, respectively; those between chicks' whole blood and rat muscle were -0.7 ± 0.1‰ and 1.7 ± 0.1‰, respectively. The mean Δ13C and Δ15N values between juvenile condors' plasma and calf muscle were 0.9 ± 0.2‰ and 3.3 ± 0.7‰, respectively; those between juveniles' red blood cells and calf muscle were 0.3 ± 0.3‰ and 1.8 ± 0.1‰, respectively; and those between juveniles' whole blood and calf muscle were 0.3 ± 0.3‰ and 1.9 ± 0.2‰, respectively. We report the first discrimination factors for the Cathartidae (New World vultures), and our findings will have important applications in studies of the critically endangered California Condor's foraging ecology. © The Cooper Ornithological Society 2013.