CONICET | Buscador de Institutos y Recursos Humanos

It is widely assumed that colonizing species thrive because they lack natural enemies in their new range, increasingtheir survival and reproductive success. Barn Swallows (Hirundo rustica) started to breed in South America around 1980and since then have dramatically increased their population size and geographic range, in stark contrast to the declineof the source population in North America. The reasons behind the growth of the South American population areunknown. However, because this species had never bred in this area in recorded times, the lack of natural predators,parasites, or pathogens could lead to higher breeding success, as predicted by the enemy release hypothesis. Here, wetest whether breeding success is higher in the newly colonized range than in the native range to test the enemy releasehypothesis. We studied the breeding biology of South American Barn Swallows quantifying 7 breeding parameters:clutch size, overall breeding success, offspring mortality by predation and ectoparasites, number of fledglings perbreeding attempt, number of breeding attempts per breeding season, and total number of fledglings produced per pairper breeding season. Additionally, we compared these parameters with published information from North Americanpopulations using meta-analyses. We found that, while clutch size did not differ between North and South Americanpopulations, the southern population overall had lower breeding success, with higher mortality from predation andectoparasites. An egg laid in South America was 2.6 times more likely to fail than one laid in North America, whichresulted on average in 1.1 fewer fledglings per pair per breeding season for the South American breeding population.These results, demonstrating lower breeding success in a newly colonized range, do not support the enemy releasehypothesis, and indicate that the growth of the South American Barn Swallow population is most likely caused by otherdemographic factors.