Females are more determinant than males in reproductive performance in the house sparrow Passer domesticus

Which sex has a more determinant role in reproductive performance? This is a long standing question in evolutionary ecology. Theory predicts and several pieces of evidence have indicated that females are more determinant than males. However, a direct test of this hypothesis has never been made. Here we perform such a direct test by using an alternative analytical approach, controlling for age and/or individual experience, and a captive population of the house sparrow Passer domesticus, a monogamous bi-parental dimorphic species in which both sexes contribute with approximately equal amounts of parental care. In this species, it has been demonstrated that contribution by males to offspring parental care is crucial to maximize reproductive success. Captive conditions with absence of predators and availability of food ad libitum allow us to control several important factors that may affect reproductive performance, such as environmental fluctuations in resource availability and differences in territory quality, without affecting reproductive performance. We consider four types of pairs, and our main prediction derived from the hypothesis is that mixed pairs involving an experienced female (FBM x EF) should present better reproductive performance than mixed pairs involving a first-time breeder female (EM x FBF). Our results confirm the general pattern reported in other bird species that older or more experienced individuals enjoy a higher reproductive success than first-time breeders. In addition, in agreement with our main prediction, we found that FBM x EF produced significantly more fledglings and had higher fledging success than EM x FBF. Our results, for the first time demonstrate the broadly accepted statement that females have a more determinant role than males in reproductive performance.