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

Abstract

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.

Removal of older males increases extra-pair siring success of yearling males

“…In this context, the vacancies created by the removal of adult males may have enhanced the social network position of yearling males and may have allowed them to gain a territory earlier in the season. In addition, if the presence of adult males increases the threat of losing paternity in the yearling males’ own brood [ 9 ], adult removal may have relieved yearling males from having to invest as much in paternity protection behaviors such as territory defense or mate guarding [ 97 ], thereby allowing them to invest more in seeking extra-pair copulations. For yearlings, the payoff of investing in extra-pair behavior may thus vary depending on the age composition of the male population, in particular, if paternity loss is age related.…”

“…For yearlings, the payoff of investing in extra-pair behavior may thus vary depending on the age composition of the male population, in particular, if paternity loss is age related. Although a higher risk of paternity loss for yearlings compared to adults has been reported in some species [ 7 , 9 ], this is not the case for the blue tit [ 9 ].…”

“…Thus, even if adult and yearling males have a similar likelihood of obtaining extra-pair copulations, adult males’ copulations may be more likely to lead to fertilizations. However, in our blue tit population, yearling and adult males with the same number of extra-pair mates have similar fertilization success [ 9 ]. Nevertheless, we cannot exclude that postcopulatory effects contribute to the age class-related increase in extra-pair siring success.…”

“…In the blue tit, females play an active role in extra-pair copulations [ 57 ]. If females select or seek extra-pair copulations from the most attractive individuals among the available males in their neighborhood, adult males may be preferred under natural conditions [ 6 , 8 , 9 , 54 , 56 ]. When adults are removed, the likelihood that a yearling male becomes the most attractive extra-pair partner among the available males may increase.…”

“…To distinguish between the 2 hypotheses, we removed almost all adult males from a population of blue tits ( Cyanistes caeruleus ) in 2022, and we compared the frequency and patterns of extra-pair paternity in the experimental year with data from the same blue tit study population during 15 “control” years (2007 to 2021). Earlier work showed a strong age effect on extra-pair siring success in this species in general [ 82 – 85 ] and also in this particular population [ 9 , 86 ].…”