Nearly 20% of bird species are migratory, their seasonal movements causing a redistribution of bird diversity that radically changes avian community composition worldwide. And yet, bird migration has been largely ignored in studies of global avian biodiversity. My thesis is the first global macroecological analysis of bird migration. Using a dataset on the geographical distributions of the world’s birds, I mapped global patterns of migratory bird diversity, revealing strong spatial features. I then used these patterns to test hypotheses exploring the ecological processes driving bird distributions in space and time. From these results, I developed the first mechanistic, process-based model of bird migration at the global scale. From first principles – converting key ecological processes into a common currency of energy – this model successfully explains the global patterns previously described. My research provides a better understanding of why birds migrate and opens new research avenues on this remarkable phenomenon.