Abstract

Switching migratory behavior appears to promote speciation in birds through colonization of their wintering grounds for breeding. However, colonization episodes often fail, especially when they lead to inbreeding depression associated with small population sizes. We studied the dynamics of a new and thriving breeding population of migratory Barn Swallows (Hirundo rustica erythrogaster) that began nesting on their traditional wintering grounds in South America in 1980 and evolved an inverse migration and breeding cycle. We show that in four decades, behavioral flexibility and standing genetic variation have facilitated the establishment of a breeding population that has grown exponentially in abundance and breeding range size. By breeding under bridges and culverts, Barn Swallows exploited ecological opportunities that outweighed the costs of inbreeding and began an independent evolutionary trajectory from their North American relatives.