Differences in timing of migration and response to sexual signalling drive asymmetric hybridization across a migratory divide.

Ecological traits and sexual signals may both contribute to the process of ecological speciation. Here we investigate the roles of an ecological trait, seasonal migratory behaviour and a sexual trait, song, in restricting or directing gene flow across a migratory divide in the Swainson's thrush (Catharus ustulatus). We show that short-distance migratory ecotypes wintering in Central America arrive earlier at the breeding grounds than long-distance migratory ecotypes wintering primarily in South America, providing the potential for some premating isolation. Playback experiments suggest that early- and late-arriving forms recognize each other as competitors, but that the early-arriving form responds more aggressively to a broader spectrum of stimuli. Genetic analysis suggests that hybridization occurs more often between males of the early-arriving ecotype and females of the late-arriving ecotype. Together our results suggest that differences in arrival times may reduce the temporal coincidence of mate choice, but asymmetry in response to heterotypic song may hinder complete divergence. These data provide further insight into the roles of ecological traits and sexual signals during the incipient stages of speciation.