Pair identity — An important factor concerning variation in extra-pair paternity in the coal tit (Parus ater)

Abstract In the majority of socially monogamous bird species, extra-pair paternity (EPP) has been shown to be common, while its frequency varies considerably among as well as within species. Our knowledge of factors affecting this variation, however, still remains comparatively poor. A thorough analysis of EPP patterns on an individual level may lead to a better understanding of the general diversity in EPP. Here, we analyse intra- and inter-annual as well as individual variation in the occurrence of EPP in first and second broods of coal tits (Parus ater) in three successive years. Based on a sample of 483 broods with 3559 offspring genotyped, we show that EPP was extraordinarily frequent in the study population, surpassing the values observed in other species of the genus Parus by far. On the population level, the rates of EPP were found to be remarkably similar between years, while there was pronounced intra-annual variation such that EPP rates increased significantly from first to second broods. Considering consecutive broods of individual females and males, the general occurrence of EPP was 'inconsistent' (i.e. EPP often affected only one of two broods), but the proportion of extra-pair young (EPY) per brood showed significant repeatability for both sexes in case of mate retention. When mate change occurred, repeatability collapsed, indicating that pair identity was more important in determining the repeatability of EPP than female or male identity alone. This was further supported by the fact that, besides female and male age, also their interaction (reflecting pair age combination) significantly predicted the proportion of EPY, though not in all breeding periods surveyed. Hence, the identity of the breeding pair (reflecting possible interactions of male and female characteristics) should be explicitly considered and accounted for in future studies investigating patterns of EPP on an individual level. Finally, we show that, depending on the boundary conditions, in principle, all coal tits experience EPP: When analysing three or more broods of the same individual, there were no completely 'faithful' females and also nearly no males, which were not cuckolded at least once.