Seasonal Stability and Immune-Related Flexibility in the Functional Structure of Passerines From Central Chile

Understanding how multiple physiological and ecological traits combine to shape functional profiles is essential for predicting how wild animals respond to environmental variability. We integrated morphological (body mass), trophic (δ^{1 3}C, δ^{1 5}N), and immune (hemagglutination, hemolysis) traits in nine passerine species sampled in summer and winter in central Chile to quantify multivariate functional profiles and assess their seasonal stability, phylogenetic structure, and environmental sensitivity. Principal component analysis identified three main axes of functional variation: a trophic-immune gradient (PC1), a body size-associated axis with secondary trophic contribution (PC2), and natural antibody-mediated immunity (PC3). Hierarchical clustering revealed a primary functional partition, whereas deeper subdivisions received limited bootstrap support, indicating that organization in functional space is better described as continuous trait gradients rather than discrete functional groups. Linear mixed-effects models using individual PC scores showed that seasonal shifts were most evident along PC1, whereas PC2 primarily reflected species-level differentiation associated with body size and dietary guild. PC3 exhibited moderate interspecific variation but limited seasonal responsiveness. Phylogenetic signal was not detected for any principal component axis, suggesting limited evidence for phylogenetic structuring of functional variation within this assemblage. Together, these results indicate that functional organization in these passerines reflects persistent species-level ecological strategies modulated by axis-specific seasonal plasticity. Integrating trophic and immune traits within a multivariate framework provides insight into how physiological differentiation and ecological specialization jointly shape community-level functional structure.