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

Abstract

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.

An experimental study of trade‐offs between brood size and moult in the blue tit (Cyanistes caeruleus)

“…1992, Arct et al . 2021). This has also been found in our population and in two other Blue Tit populations in Norway (Krokene & Lifjeld 2000, Johannessen et al .…”

“…In a study of Blue Tits in Sweden, fewer cases of extra‐pair paternity occurred in broods when old males were paired with old females but not when old males were paired with young females (Arct et al . 2021).…”

“…In Blue Tits, annual survival rate is low (usually about 20-60%; Perrins 1979, Slagsvold & Lifjeld 1990, Dhondt et al 1998, Grosbois et al 2006) and even if the male survives, his paternity may not be more certain in a subsequent brood. In a study of Blue Tits in Sweden, fewer cases of extra-pair paternity occurred in broods when old males were paired with old females but not when old males were paired with young females (Arct et al 2021).…”

“…In Blue Tits, about 30-40% of the broods may contain at least one nestling sired from extrapair copulation (e.g. Kempenaers et al 1992, Arct et al 2021). This has also been found in our population and in two other Blue Tit populations in Norway (Krokene & Lifjeld 2000, Johannessen et al 2005.…”

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

“…In our population, on average, 46% of all individuals in the population are yearlings (range: 28%–61%, Figure S2). Blue tits are socially monogamous with occasional social polygyny (Schlicht & Kempenaers, 2021) and frequent EPP (Arct et al, 2022; Badás et al, 2020; Delhey et al, 2003; Kempenaers et al, 1997; Mennerat et al, 2018; Vedder et al, 2011). In our population, each year on average 43% of nests contain EPY (range 35%–53%) and 10% of young are not sired by their social father (range 8%–15%; see Table S1 for basic metadata and contextual information regarding parentage).…”