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Abstract 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 data sets 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. 

Mating with another species is often maladaptive because it generally results in no or low-fitness offspring. When hybridization is sufficiently costly, individuals should avoid mating with heterospecifics even if it reduces their ability to mate with high-quality conspecifics that resemble heterospecifics. Here, we used spadefoot toads, Spea multiplicata, to evaluate whether females alter their preferences for conspecific male sexual signals (call rate) depending on heterospecific presence. When presented with conspecific signals against a background including both conspecific and heterospecific signals, females preferred male traits that were most dissimilar to heterospecifics—even though these signals are potentially associated with lower-quality mates. However, when these same females were presented with a background that included only conspecific signals, some females switched their preferences, choosing conspecific signals that were exaggerated and indicative of high-quality conspecific mates. Because only some females switched their preferences between these two chorus treatments, there was no population-level preference for exaggerated conspecific male signals in the absence of heterospecifics. These results show that hybridization risk can alter patterns of mate choice and, consequently, sexual selection on male signals. Moreover, they emphasize that the strength and expression of reproductive barriers between species (such as mate choice) can be context-dependent.