Age trajectories in extra-pair siring success suggest an effect of maturation or early-life experience

Across birds, male age is the most consistent predictor of extra-pair siring success, yet little is known about age effects on paternity over the lifetime of individuals. Here, we use data from a 13-year study of a population of blue tits (Cyanistes caeruleus) to investigate how extra-pair siring success changes with age within individuals. Our results indicate that extra-pair siring success does not continuously increase with male age. Instead, siring success was related to male age in a threshold fashion, whereby yearling males were less likely to gain paternity than older males. This effect was independent of the age of the social partner, but influenced by the age of the extra-pair female: success of yearlings at siring extra-pair young (EPY) with older females was even lower. Among males that sired at least one EPY, the number of extra-pair mates and the proportion of EPY sired were unrelated to male age. We found no evidence for an influence of selective disappearance on extra-pair reproduction. Senescence, if anything, only occurs at ages blue tits rarely reach. A literature review indicates that an effect of male age on extra-pair siring success may be limited to the switch from yearling to older in many species. Thus, the generally observed age effect on male extra-pair siring success may be linked to age class rather than continuous ageing. This suggests that lack of experience or not fully completed maturation are important drivers of age patterns in extra-pair paternity.