No Consistent Effect of Migration on Speciation Rates in Two Avian Superfamilies: A Check on the Robustness of Trait-Dependent Diversification Methods

Seasonal migration is performed by taxonomically diverse groups across the planet's oceans and continents. Migration has been hypothesized to promote speciation through a variety of mechanisms that may initiate reproductive isolation and population divergence, such as temporal or spatial migratory divides, migration 'falloffs', or the colonization of new, geographically isolated breeding areas. Migration has also been implicated in recent population divergence within a handful of bird species; however, it is unknown whether migration is generally associated with higher speciation rates. We sought to test this question in two large clades of new world birds with diverse migratory phenotypes, the suboscines and the Emberizoidea, employing three state-of-the-art comparative methods of trait-based diversification: estimates of tip speciation rates using 1) BAMM and 2) ClaDS; and 3) hidden-state speciation extinction models. Our results differed across methods and across taxonomic scales, suggesting an acute need to corroborate inferences across different frameworks and datasets prior to concluding that a given trait has, in fact, promoted diversification. Overall, and based upon the majority of results across different methods, we conclude that there is no methodologically-consistent evidence of faster speciation in migratory lineages in these groups. We discuss the biological implications of this finding, as well as the challenges of inference posed by current trait-based diversification methods.