Abstracts (first author)
Hybrid songbirds employ intermediate routes in migratory divide
Seasonal migration may play a significant role in speciation; many divergent populations breed adjacent to one another while using different routes to reach their wintering grounds (i.e., form migratory divides). Migratory orientation is largely genetically determined in these populations and often involves navigation around geographic barriers. Accordingly, hybrids have been predicted to employ intermediate routes that are inferior to those of parental forms. We provide the first direct test of this hypothesis here, by attaching light-level geolocators to birds in a narrow hybrid zone between two groups of Swainson’s thrushes that form a migratory divide in western North America. Most of these birds employed intermediate routes, navigating over large geographic barriers. The remainder employed a mixed strategy, using the same route as one parental form on fall migration and the other on spring migration. Data from age ratios and cline analyses further suggests that hybrids survive at lower rates than parental forms and are selected against. Together, these results provide strong support for the migratory divide hypothesis and represent one of few established examples in which a behavioral trait reduces hybrid fitness, thereby promoting speciation.