Abstract
Within breeding years, male birds vary in their reproductive strategy. While some maintain monogamy with a social partner, others also engage with extrapair partners, while others forgo monogamy altogether in favour of exclusively seeking extrapair paternity. Although theory predicts that extrapair paternity is beneficial to males, which sire extrapair offspring without investing in costly parental care, empirical examples from wild populations are sparse. We used 17 years of data from a closed population of house sparrows, Passer domesticus, with a complete genetic pedigree, to test the hypothesis that extrapair paternity increases male lifetime reproductive success. We compared a mixed strategy of within-pair (or social) and extrapair paternity with total genetic monogamy and total extrapair paternity. We demonstrate that males who combine within-pair and extrapair paternity have increased reproductive success against the other two groups. Our results also suggest that males that exclusively seek extrapair paternity have the lowest lifetime fitness. Overall, we provide an empirical demonstration of the theory, showing that where males can sire extrapair offspring alongside within-pair offspring, extrapair paternity is beneficial to male lifetime fitness.
•Within years, male house sparrows invested in social or extrapair offspring, or both.•Within a year, reproductive strategy depended on the environmental context.•Males that had extrapair and social offspring had the highest lifetime fitness.•Floating males, which only had extrapair offspring, had the lowest lifetime fitness.