Assessing Avian Diets Using Stable Isotopes II: Factors Influencing Diet-Tissue Fractionation

Studies using stable-isotope analysis to infer diet require a knowledge of how stable-isotope ratios in consumer tissues are related to dietary values. We determined 61'3C and 6'5N diet-tissue fractionation factors for blood, liver, muscle, bone collagen and feathers of domestic chickens (Gallus gallus), Japanese Quail (Coturnix japonica) and Ring-billed Gulls (Larus delawarensis) and for blood and feather samples of adult Peregrine Falcons (Falco peregrinus) raised on known isotopic diets. In most cases tissues were enriched in 613C and 615N compared to diet. However, fractionation values differed among species, diets, and tissue types and this variation must be considered in isotope dietary studies. We found little evidence that isotopic fractionation factors are influenced by age in adult birds and provide fractionation factors appropriate for granivores feeding in a C-3 biome and for higher trophic-level piscivores and carnivores. American Crows (Corvus brachyrhynchos) raised on a perch diet grew faster and showed lower nitrogen diet-tissue fractionation values than crows raised on a plant-based diet. We suggest that nutritional stress caused substantial increases in diet-tissue fractionation values due either to: (1) mobilization and redeposition of proteins elsewhere in the body; or (2) amino acid composition changes in tissues.