Tracing sources of carbon and hydrogen to stored lipids in migratory passerines using stable isotope (δ13C, δ2H) measurements

Using measurements of naturally occurring stable isotopes in feathers to determine avian origin and migratory patterns is well established. However, isotopically determining nutritional origins of lipids, a major migratory fuel, has not been attempted. This study explores isotopic links between diet and stored lipids in captive white-throated sparrows (Zonotrichia albicollis) by providing isotopically distinct mixtures of carbohydrates/oils and drinking water and assessing the delta C-13 and delta H-2 values of stored lipid, breath CO2 (delta C-13) and breath water vapour (delta H-2). Stored lipid delta C-13 and delta H-2 values correlated with the isotopic values found in dietary carbohydrates/oils and drinking water treatments, respectively, indicating a clear traceable transfer of environmental dietary isotopic signals into body lipids. Dietary oils and carbohydrates contributed 80-82% of carbon and 44-46% of hydrogen, respectively, to stored lipids. Drinking water contributed 18-28% of hydrogen to stored lipids. Isotopic relationships were quantifiable using linear calibration algorithms which provide the basis for the construction of tissue isoscapes for migratory passerines. Breath CO2 delta C-13 values and breath water vapour delta H-2 values for fed and fasted birds reflected dietary sources. Breath CO2 delta C-13 values were higher for fasted birds than for fed birds by an average of 4.5 parts per thousand while breath water vapour delta H-2 values were lower for fasted birds by an average of 48.9 parts per thousand. These results indicate that lipids and metabolites from their subsequent breakdown for fuel isotopically reflect dietary sources but complicate interpretation of such data, especially for wild migrating birds. Applications and limitations of these findings to the creation of "liposcapes" are examined.