Data from: A migratory divide spanning two continents is associated with genomic and ecological divergence

Migratory divides are contact zones between breeding populations with
divergent migratory strategies during the non-breeding season. These
locations provide an opportunity to evaluate the role of seasonal
migration in the maintenance of reproductive isolation, particularly the
relationship between population structure and features associated with
distinct migratory strategies. We combine light-level geolocators, genomic
sequencing, and stable isotopes to investigate the timing of migration and
migratory routes of individuals breeding on either side of a migratory
divide coinciding with genomic differentiation across a hybrid zone
between barn swallow (Hirundo rustica) subspecies in China. Individuals
west of the hybrid zone, with H. r. rustica ancestry, had comparatively
enriched carbon isotope values and overwintered in eastern Africa, while
birds east of the hybrid zone, with H. r. gutturalis ancestry, had
depleted isotope values and migrated to southern India. The two subspecies
took divergent migratory routes around the high-altitude Karakoram Range
and arrived on the breeding grounds over three weeks apart. These results
indicate that assortative mating by timing of arrival and/or selection
against hybrids with intermediate migratory traits may maintain
reproductive isolation between the subspecies, and that inhospitable
geographic features may have contributed to the diversification of Asian
avifauna by influencing migratory patterns.