Barn Swallows colonize South America
Why scientists believe Barn Swallows breeding in Buenos Aires Province, Argentina, may represent not only a range expansion but a rare colonization of a continent.
Published: October 21, 2011
Familiar fork-tailed Barn Swallow is the most widely distributed and abundant swallow in the world. It nests across Canada, throughout most of the United States, and in northern Mexico and much of Europe and Asia.
Until recently, it was thought only to spend the winter in South America, not to breed there. So it was big news in 1980 when six pairs were found nesting in Buenos Aires Province, Argentina, at least 1,000 kilometers south of where most North American swallows winter.
The birds’ pioneering behavior was unprecedented, and it sparked hopes that scientists would get to document a rare occurrence: a range expansion and the colonization of a continent.
Three decades later, researchers appear to have gotten their wish.
Not only is the South American breeding population well established — it now includes thousands of pairs — but the increase is occurring without the cascade of genetic changes predicted by evolutionary theory.
Shawn Billerman and colleagues compared the genetic structure of swallows from Argentina and North America and determined that the Argentine population has likely received, and may still be receiving, substantial gene flow from birds of North American origin.
That is, Barn Swallows may be hatching up north, overshooting their South American winter grounds, and then shifting their breeding and molt cycles by six months to settle into an austral breeding pattern.