Phylogenetic comparative analysis of functional morphology reveals the origin of long-distance migration in nightingale-thrushes (Turdidae: Catharus)

This study investigates the evolution of locomotory morphology and migratory behavior in nightingale-thrushes (genus Catharus), a clade of songbirds with diverse migratory strategies. With large datasets of molecular and morphometric characters, we resolve phylogenetic relationships, identify and model migration-related morphological characters, and estimate ancestral states of those characters to infer evolutionary transitions in the migratory phenotype. Our results indicate that (1) migratory behavior and its functional morphology are fundamentally linked and can be accurately modeled along a linear axis; (2) long-distance migration was not the ancestral state of Catharus, contradicting some previous findings; (3) short distance and elevational migration were precursors to long distance migration; and (4) the apparent homoplasy of the migratory phenotype, as noted by previous authors, was likely caused by successive "budding" of daughter lineages from a persistent "stem" (ancestral) lineage, not evolutionary convergence following independent origins of migration. Our data also suggest that the number of species in Catharus is considerably underestimated, setting the stage for a future taxonomic revision.