Natural selection and circular pathways to seasonal migration in birds

The "migratory revolutions" (MR) model is a synthetic theory of bird migration that seeks to explain the full range of the functional phenotype, from sedentary residents of non-seasonal (tropical) habitats to obligate long-distance migrants, as a cumulative evolutionary response to shifting distributions of adult extrinsic mortality across the annual cycle. At macroevolutionary scales, the general model predicts that migration evolves in circular patterns, reframing classic debates about the effects of migration on speciation and extinction rates. Here, I describe and apply the MR model to a well-known system, the passerine genus Catharus (Turdidae), to illustrate its broad implications for reconstructing evolutionary history.