Stable isotope analysis reveals dietary and foraging ecology in Neotropical birds

Abstract Birds, as highly mobile organisms, exploit a wide variety of resources and play vital roles in ecosystem functioning. Because individuals of the same species typically share dietary preferences, stable isotopes are expected to reflect their trophic niches closely. We tested how well phylogenetically informed traits, including dietary guild and foraging stratum, explain isotopic variation among Neotropical birds. Our dataset included δ1³C and δ15N values from 1,330 feather samples representing 138 species across 23 families, collected in 3 major Brazilian biomes: the Cerrado savanna, the Amazon, and the Atlantic Forest. Dietary guild and stratum both explained significant isotopic variation, but stratum emerged as an especially important predictor of both δ1³C and δ15N values. Granivores exhibited less negative δ1³C values, indicating a larger reliance on C4 plants, such as pasture grasses. Frugivores showed more negative δ1³C values, consistent with a diet dominated by C3 plants. Invertivores and omnivores had elevated δ15N values, reflecting higher animal protein intake. The strong isotopic segregation between strata (e.g., terrestrial vs. arboreal–insessorial) further highlighted the role of vertical foraging strata in shaping bird diets. Bayesian isotopic mixing models revealed that granivores primarily consumed C4 plants (∼60%), while frugivores and omnivores incorporated both C3 and C4 resources. Importantly, δ15N values increased with human disturbance, suggesting shifts in food web structure or trophic position. Our findings underscore the value of stable isotope analysis in linking ecological traits to resource use and emphasize the importance of foraging behavior in shaping isotopic patterns, with implications for monitoring biodiversity and habitat integrity