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, but 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‐rumped 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.