Metabolic Routing of Dietary Nutrients in Birds: Effects of Dietary Lipid Concentration on δ13C of Depot Fat and Its Ecological Implications

During migration, many songbirds eat primarily fruit while depositing fat. Given that some fruits contain mostly carbohydrate and others contain mostly lipid, the ability of birds to fatten may depend on the macronutrient composition of the fruit. Stable isotopes of carbon may be useful in determining the source of nutrients used for synthesizing fat, because the enzyme that regulates the transfer of carbon skeletons from carbohydrate into fat synthesis has a higher affinity for 12C than for 13C, whereas dietary lipids can be directly incorporated into animal fat. Thus, fat stores of animals that are synthesized directly from dietary lipid should have isotopic signatures similar to dietary lipid, whereas biosynthesis of fats from dietary carbohydrates should produce changes in isotopic signatures. We tested these predictions by manipulating the concentrations and isotopic signatures of macronutrients in diets fed to Yellow-rumped Warblers (Dendroica coronata). The δ13C of depot fat in birds fed high-lipid diets was similar to that of dietary lipid, whereas δ13C of depot fat in birds fed low-lipid diets indicated that a combination of dietary lipid and carbohydrate were used to synthesize depot fat. Models that incorporated 8% discrimination between dietary carbohydrate and depot fat consistently estimated the proportion of dietary lipid and carbohydrate routed into depot fat. Stable-isotope analysis of macronutrients in the diet of wild birds combined with estimates of the effects of diet composition on the isotopic signature of depot fat in birds offer a method to identify the relative importance of nutritional resources used by songbirds to deposit fat. Rutas Metabólicas de Nutrientes Dietarios en Aves: Efectos de la Concentración de Lípidos Dietarios sobre δ13C en los Depósitos de Grasa y sus Implicancias Ecológicas

Fruit Abundance and Local Distribution of Wintering Hermit Thrushes (Catharus Guttatus) and Yellow-Rumped Warblers (Dendroica Coronata) in South Carolina

We conducted winter censuses of two short-distance migrants, Hermit Thrushes (Catharus guttatus) and Yellow-rumped Warblers (Dendroica coronata), over seven years in five different habitats to determine whether their local abundances could be predicted by fruit pulp biomass. Sampled habitats were stands of upland and bottomland hardwood, loblolly pine (Pinus taeda), longleaf pine (P. palustris), and young (<10 years) longleaf pine. Hermit Thrush abundance, which was highest in bottomland hardwood habitats, was positively related to total dry mass of fruit pulp. Those results are consistent with the hypothesis that resource availability affects the local distribution of migrant passerines on their wintering grounds. Our results also indicate that bottomland hardwood habitats in the southeastern United States may be especially important to wintering Hermit Thrushes. Yellow-rumped Warbler abundance was correlated with ripe-fruit pulp dry mass of Myrica cerifera, a major source of winter food for that species. However, because M. cerifera pulp dry mass was confounded with habitat type, we could not distinguish the relative importance of fruit resources and habitat for Yellowrumped Warblers. Our results underscore the importance of fruit to wintering birds. However, the overall percentage of variation in winter bird abundance explained by differences in ripefruit biomass was modest, indicating that other factors are also important.