In this study, we investigated the relationship between blood IGF-1 concentrations, body mass, and lifespan within natural bird populations through comparative analysis using phylogenetic generalized least squares (PGLS) models. Our research comprised 92 bird species spanning various Orders. We have used an in-house developed competitive ELISA in determining the IGF-1 concentrations, which was improved before the beginning of this research. We found no relationship between IGF-1 and longevity while showing a positive interaction between IGF-1 and body mass. We also demonstrated a positive association between body mass and longevity.