Discrimination Factors for Stable Isotopes of Carbon and Nitrogen in Blood and Feathers from Chicks and Juveniles of the California Condor

Stable-isotope ratios of carbon (C-13/C-12; delta C-13) and nitrogen (N-15/N-14; delta N-15) 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 Delta), vary considerably by species and tissue. Variations in discrimination factors used in stable-isotope mixing models can lead to inaccurate estimates of diets. Therefore, determining accurate discrimination factors specific to species and tissue is important. We established the Delta C-13 and Delta N-15 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 Delta C-13 and Delta N-15 values (+/- SD) between chick feathers and rat muscle were 0.4 +/- 0.4 parts per thousand and 3.1 +/- 0.2 parts per thousand, respectively; those between chicks' whole blood and rat muscle were -0.7 +/- 0.1 parts per thousand and 1.7 +/- 0.1 parts per thousand, respectively. The mean Delta C-13 and Delta N-15 values between juvenile condors' plasma and calf muscle were 0.9 +/- 0.2 parts per thousand and 3.3 +/- 0.7 parts per thousand, respectively; those between juveniles' red blood cells and calf muscle were 0.3 +/- 0.3 parts per thousand and 1.8 +/- 0.1 parts per thousand, respectively; and those between juveniles' whole blood and calf muscle were 0.3 +/- 0.3 parts per thousand and 1.9 +/- 0.2 parts per thousand, 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.