Ecological opportunities and individual condition as predictors of extra-pair paternity in a south-temperate swallow (Tachycineta leucorrhoa)

Ecological and physiological factors such as breeding density, breeding synchrony, and adult body condition can all affect extra-pair mating behavior, bur the relative importance of these factors may vary among species. White-rumped Swallows (Tachycineta leucorrhoa) nesting in Buenos Aires Province, Argentina, exhibit high rates of extra-pair paternity, with 77% of nests having extra-pair young. Our objective was to determine the extent to which extra-pair paternity in this species is explained by breeding synchrony, breeding density, and adult body condition. Our study of a population of White-rumped Swallows breeding in nest boxes was conducted during two consecutive breeding seasons (September - early January 2006-2008). We found that neither breeding synchrony nor density of neighbors predicted levels of extra-pair paternity in our study population. Leaner females were more likely to engage in extra-pair behavior and fledged more nestlings, but did not differ in structural size from females that did not engage in extra-pair behavior, suggesting that female mass is an important predictor of mating decisions and fitness for these aerial insectivores. Male body condition was not related to male extra-pair behavior. The mass of female White-ramped Swallows may affect their flying ability such that, during their fertile period, they are exposed to more potential extra-pair mates during longer foraging flights. Being lighter may also improve the ability of females to provision nestlings later in the breeding cycle.