Food supplementation affects extrapair paternity in house sparrows (Passer domesticus

Extrapair paternity (EPP) is common among birds, but the reasons why it varies within and among species are less clear. In particular, few studies have experimentally examined how food availability influences paternity and sexual behavior. We manipulated food supply in a nest-box population of house sparrows, Passer domesticus, a colonial passerine with extensive biparental care. During three successive breeding attempts, we changed food availability at nest sites and examined behavior and genetic parentage. DNA fingerprinting revealed that the level of EPP within broods was five times lower in pairs nesting at sites continuously supplied with extra food. With extra food, mates spent longer time together at the nest, but this was mainly due to a change in female behavior; females but not males increased total nest attendance. Moreover, we found that individual males did not change within-pair copulation frequency across treatments, suggesting that our experiment did not influence male control over fertilizations through copulation behavior. Instead, our study shows that ecological factors can have a strong influence on the time budgets of males and females, which consequently affects the occurrence of EPP. Key words: extrapair paternity, food supplementation, female behavior, house sparrows, Passer domesticus. [Behav Ecol 14:730–735 (2003)] There is a considerable variation in the frequency ofextrapair paternity (EPP) within as well as among bird species, but the reasons for this variation remain largely unclear (reviewed by Petrie and Kempenaers, 1998). Socio-ecological factors such as breeding synchrony (Stutchbur