Common paths link food abundance and ectoparasite loads to physiological performance and recruitment in nestling blue tits

Summary1 Identifying how selection shapes life‐history traits by causally relating environment to phenotype, performance and fitness has often proven elusive due to limitations of classical analysis methods, which only identify covariance in traits, and to the difficulties in experimentally manipulating environment to expose cause and effect in wild populations. 2 In an approach resembling the experimental method common to all modern research, structural equation modelling can not only identify covariance in traits, but also test hypotheses of direct and indirect causal paths that tie environment to phenotypes and fitness through natural selection. Here, we use novel confirmatory path analyses and a long‐term study of Corsican blue tits to analyse the interactions between environmental variables (prey abundance and ectoparasite load), phenotypic traits (mass and haematocrit), physiological performance (aerobic capacity) and nestling recruitment. 3 Our analyses show that an antagonistic interaction between ectoparasites and food abundance sets tissue development and oxygen carrying capacity of blood at fledging, and that identical paths link these variables to physiological performance and recruitment. 4 This study suggests that metabolic capacity at fledging may be important in determining subsequent recruitment and unmasks subtle fitness costs of an ectoparasite.