Functional relations between body mass and risk-taking behavior in wild great tits

Natural selection often favors particular combinations of functionally-related traits, resulting in adaptive phenotypic integration. Phenotypic integration has been proposed as a potential mechanism explaining the existence of repeatable among-individual differences in behavior (i.e., animal personality). In this study, we investigated patterns of covariation between morphology and behavior in a population of free-living great tits (Parus major) monitored for 7 years. In particular, we aimed to disentangle the effect of structural size versus body condition on risk-taking behavior. To do so, we repeatedly quantified multiple morphological (body mass, wing, tarsus, and bill length) and behavioral traits (aggressiveness and exploration) in 742 individual males. Structural equation modeling (SEM) allowed us to test causal a priori hypothesized relationships between the different morphological and behavioral traits. Our best-fitting SEM model supported the existence of a behavioral character, risk-taking behavior that covaried simultaneously with the latent variable body size, and body condition. Our findings thus demonstrate that an individual's morphological and behavioral traits represent expressions of an integrated phenotype, suggesting a role for phenotypic integration in generating animal personality in a wild bird population. Many studies show that individual animals are repeatable in their behavior, but are these differences part of whole-organism differences? Using long term data from a wild population of great tits, we show that body size and body condition differences were related to differences in risk-taking behavior (exploration and aggression) but had opposing effects, thereby providing an explanation of why we observe long-standing individual differences in behavior in the wild.