Barn Swallows keep expanding their breeding range in South America

Barn Swallows (Hirundo rustica) breeding in North America traditionally wintered in South America. In the 1980s a small breeding population was found in coastal Argentina and since then the breeding range of Barn Swallows in the country has expanded. Our aims here were to examine the north-western limits of the breeding range of Barn Swallows in Argentina, and to analyse the factors that may determine selection of nesting site by the species. We surveyed and characterised culverts and bridges below roads, which are the most frequently used breeding sites of Barn Swallows in Argentina, in an area of 15 000 km2 in northern La Pampa Province, central Argentina. We found 51 nests in 39 culverts and bridges sparsely distributed through most of the surveyed area. We also found 16 nests outside that area, in central Buenos Aires and La Pampa Provinces. Occupied culverts were higher and were more often associated with wetlands than unoccupied culverts. Most nests were in agricultural landscapes, with just a few in Calden (Prosopis caldenia) forests. Our results confirm that the breeding range of the species in South America has expanded by about a third in relation to previous studies, but there is still a large area of suitable habitat for the species in Argentina, mostly to the north of La Pampa Province, suggesting that further expansion of the range of the species can be expected. Our study describes the expansion of this common species within South America and speculates on the underlying reasons.