Extra-pair paternity in Blue Tits (Cyanistes caeruleus) depends on the combination of social partners' age

There is growing evidence that engaging in extra-pair copulations may be a strategy by which females can modify their initial mate choice if they are constrained by primary choice of the social mate. Several factors such as genetic similarity and adult phenotypic traits can affect extra-pair paternity (EPP) patterns, but the relative importance of these factors may vary among species. Moreover, interactive effects of male and female characteristics have rarely been considered. Here, we aimed to study how multiple parameters characterizing a breeding pair (i.e. genetic similarity between mates, partners' age and laying date) predict the occurrence of EPP at the brood level. The study uses 4 years of data from a wild population of Blue Tits Cyanistes caeruleus. Contrary to predictions of the inbreeding avoidance hypothesis, we did not find a positive relationship between the occurrence of EPP and the relatedness of social mates. We also found that the probability of EPP was unrelated to laying date. However, EPP was predicted by an interaction of social partners' ages. Specifically, EPP was less likely when old females were paired with old males in comparison to old females paired to young males. Our study suggests that the occurrence of EPP may be the result of behavioural interactions in which both male and female age are important for determining the outcome. Our results confirm the importance of considering the interactions of male and female characteristics in studies investigating EPP patterns.