Life-history and ecological variables as drivers of the evolution of avian innate immune defences

Animals rely on immune defences to counteract pathogens and parasites, but investing in immune defences limits the use of shared resources for other functions. Therefore, the strength of immune defences is hypothesized to be shaped by life history or broader ecology. To better understand ecological and evolutionary patterns in innate humoral immune defences in birds, we compiled more than 400 immunological records of more than 100 avian species from about 100 previous studies that applied a standardized protocol to measure haemolysis and haemagglutination. We extracted data on 15 life-history and ecological variables and built phylogenetically informed comparative models to determine how these variables can explain variation in both immunological indices. We also inferred evolutionary patterns by selecting the best-fitting macroevolutionary models. Our comparative models indicated that several ecological variables, including seasonal stage, sex, age, migration distance, diet type and climatic factors, and only one life-history variable, body mass, were able to explain the immunological variation. The best macroevolutionary models suggest that both immune indices evolved largely through gradual divergence under strong phylogenetic constraints, with evidence for stabilizing selection within Passeriformes. Overall, our results highlight the role of seasonal and demographic pressures, alongside phylogenetic history, in shaping immune variation across avian species.