Male age predicts extrapair and total fertilization success in the socially monogamous coal tit

An important step to a better understanding of the adaptive significance of extrapair mating behavior in socially monogamous species is to uncover the identity of extrapair sires (EPS). Here, we used a combination of multilocus DNA fingerprinting and microsatellite analysis to identify EPS in the socially monogamous coal tit (Periparus ater), a passerine bird with high rates of extrapair paternity. We then analyzed how fertilization success was related to male age in 2 consecutive first brood periods based on knowledge of the exact age of the majority of territorial males. EPS were significantly older compared with the males they cuckolded. Furthermore, extrapair and, as a consequence, also total fertilization success were positively related to male age, while within-pair success was not. Interestingly, fertilization success did not increase linearly with male age but leveled off for older age classes and was most parsimoniously described by the inverse term of male age. Results were consistent over the 2 years, while the demography of the study population differed with respect to the age distribution of territorial males. Furthermore, we also show that individual males increased their extrapair fertilization (EPF) success with age indicating that cross-sectional analyses were not confounded by cohort effects. Together with the results from other species, these findings suggest that male age (or a strong correlate thereof) is a major determinant of EPF success in several socially monogamous bird species. Copyright 2007, Oxford University Press.