Trophic position structures physiological trade-offs between metabolic and detoxification pathways in sympatric passerines.

Resource use links ecological context to organismal performance, yet whether dietary breadth or trophic position is more closely associated with physiological state in wild birds remains unclear. We used a natural trophic gradient in a sympatric passerine assemblage to test whether trophic position and isotopic niche breadth are associated with integrated physiological phenotypes. We studied three co-occurring passerine species in central Chile spanning a trophic spectrum (granivorous, omnivorous, insectivorous). Using blood δ 13C and δ 15N, we quantified isotopic niche width and estimated trophic position, then compared these metrics with physiological variables measured in free-living birds (n = 60), including differential leukocyte counts, antioxidant capacity, and enzymatic markers of oxidative metabolism, glutathione-related physiology, and plasma esterase activity. Isotopic niche width differed among species but was weakly associated with physiological variation. By contrast, estimated trophic position showed stronger species-level correspondence with multivariate physiological organization. Metabolic and enzyme-based variables contributed most to interspecific physiological differentiation, whereas baseline non-enzymatic antioxidant buffering was conserved. Overall, estimated trophic position showed a stronger correspondence with coordinated physiological variation than isotopic niche breadth. These findings highlight species-level physiological differences associated with trophic position and provide a basis for broader comparative analyses of ecological and evolutionary influences on avian physiology.